test(showcase): Add gRPC and HttpJson Showcase ITs to verify Error Details (#13928)

### Description
This PR introduces E2E integration test coverage for standard and custom
error details using the `FailEchoWithDetails` RPC in Showcase,
validating the exception propagation and deserialization pipeline across
both gRPC and HTTP/JSON transports.

### Key Changes
* **gRPC Transport Verification**: 
* Verifies unpacking and correctness of all standard error details (e.g.
`ErrorInfo`, `RetryInfo`, `DebugInfo`, `QuotaFailure`,
`PreconditionFailure`, `BadRequest`, `RequestInfo`, `ResourceInfo`,
`Help`, `LocalizedMessage`).
* Verifies that Showcase-specific custom trailers (e.g. `PoetryError`)
are successfully unpacked and mapped into the client-side
`ErrorDetails`.
* Verifies dynamic message propagation inside custom trailers when
parameters are passed to the request.
* **HTTP/JSON Transport Verification**:
* Verifies that error status code mapping (e.g. `ABORTED`) propagates
correctly.
* Handles GAX's HTTP/JSON parser boundaries when dealing with
unregistered custom types (like `PoetryError`) in JSON status payloads.

Author: nnicolee

java-showcase/gapic-showcase/src/test/java/com/google/showcase/v1beta1/it/ITErrorDetails.java

@@ -0,0 +1,261 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.showcase.v1beta1.it;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import com.google.api.client.http.HttpResponseException;
+import com.google.api.gax.rpc.ApiException;
+import com.google.api.gax.rpc.ErrorDetails;
+import com.google.api.gax.rpc.StatusCode;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParser;
+import com.google.protobuf.TypeRegistry;
+import com.google.protobuf.util.JsonFormat;
+import com.google.rpc.BadRequest;
+import com.google.rpc.DebugInfo;
+import com.google.rpc.ErrorInfo;
+import com.google.rpc.Help;
+import com.google.rpc.LocalizedMessage;
+import com.google.rpc.PreconditionFailure;
+import com.google.rpc.QuotaFailure;
+import com.google.rpc.RequestInfo;
+import com.google.rpc.ResourceInfo;
+import com.google.rpc.RetryInfo;
+import com.google.rpc.Status;
+import com.google.showcase.v1beta1.EchoClient;
+import com.google.showcase.v1beta1.FailEchoWithDetailsRequest;
+import com.google.showcase.v1beta1.PoetryError;
+import com.google.showcase.v1beta1.it.util.TestClientInitializer;
+import java.util.concurrent.TimeUnit;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Integration tests for verifying that client libraries correctly propagate and deserialize
+ * standard and custom error details from {@link ApiException} over gRPC and HTTP/JSON transports.
+ */
+class ITErrorDetails {
+
+  private static final TypeRegistry SHOWCASE_ERROR_TYPE_REGISTRY =
+      TypeRegistry.newBuilder()
+          .add(ErrorInfo.getDescriptor())
+          .add(RetryInfo.getDescriptor())
+          .add(DebugInfo.getDescriptor())
+          .add(QuotaFailure.getDescriptor())
+          .add(PreconditionFailure.getDescriptor())
+          .add(BadRequest.getDescriptor())
+          .add(RequestInfo.getDescriptor())
+          .add(ResourceInfo.getDescriptor())
+          .add(Help.getDescriptor())
+          .add(LocalizedMessage.getDescriptor())
+          .add(PoetryError.getDescriptor())
+          .build();
+
+  private static EchoClient grpcClient;
+  private static EchoClient httpjsonClient;
+
+  @BeforeAll
+  static void createClients() throws Exception {
+    grpcClient = TestClientInitializer.createGrpcEchoClient();
+    httpjsonClient = TestClientInitializer.createHttpJsonEchoClient();
+  }
+
+  @AfterAll
+  static void destroyClients() throws InterruptedException {
+    grpcClient.close();
+    httpjsonClient.close();
+
+    grpcClient.awaitTermination(TestClientInitializer.AWAIT_TERMINATION_SECONDS, TimeUnit.SECONDS);
+    httpjsonClient.awaitTermination(
+        TestClientInitializer.AWAIT_TERMINATION_SECONDS, TimeUnit.SECONDS);
+  }
+
+  /**
+   * Helper method to verify the content of error details. Transport-neutral validation of standard
+   * and custom error packets.
+   */
+  private void verifyErrorDetailsContent(ErrorDetails errorDetails, String expectedPoem) {
+    // Verify standard error details are present and populated
+    // We are assuming that the mock server's hardcoded return values will stay the same
+    // https://github.com/googleapis/gapic-showcase/blob/b6c247f153369044d599969f6929ecdeb066c4c6/server/services/echo_service.go
+    ErrorInfo errorInfo = errorDetails.getErrorInfo();
+    assertThat(errorInfo).isNotNull();
+    assertThat(errorInfo.getReason()).isEqualTo("some ErrorInfo reason");
+
+    RetryInfo retryInfo = errorDetails.getRetryInfo();
+    assertThat(retryInfo).isNotNull();
+    assertThat(retryInfo.getRetryDelay().getSeconds()).isEqualTo(11);
+
+    DebugInfo debugInfo = errorDetails.getDebugInfo();
+    assertThat(debugInfo).isNotNull();
+    assertThat(debugInfo.getDetail()).isEqualTo("a DebugInfo detail");
+
+    QuotaFailure quotaFailure = errorDetails.getQuotaFailure();
+    assertThat(quotaFailure).isNotNull();
+    assertThat(quotaFailure.getViolations(0).getDescription())
+        .isEqualTo("First QuotaFailure description");
+
+    PreconditionFailure preconditionFailure = errorDetails.getPreconditionFailure();
+    assertThat(preconditionFailure).isNotNull();
+    assertThat(preconditionFailure.getViolations(0).getDescription())
+        .isEqualTo("First PreconditionFailure description");
+
+    BadRequest badRequest = errorDetails.getBadRequest();
+    assertThat(badRequest).isNotNull();
+    assertThat(badRequest.getFieldViolations(0).getDescription())
+        .isEqualTo("First BadRequest description");
+
+    RequestInfo requestInfo = errorDetails.getRequestInfo();
+    assertThat(requestInfo).isNotNull();
+    assertThat(requestInfo.getRequestId()).isEqualTo("RequestInfo: showcase-request-id");
+    assertThat(requestInfo.getServingData()).isEqualTo("RequestInfo: showcase serving data");
+
+    ResourceInfo resourceInfo = errorDetails.getResourceInfo();
+    assertThat(resourceInfo).isNotNull();
+    assertThat(resourceInfo.getResourceType()).isEqualTo("ResourceInfo: showcase resource");
+
+    Help help = errorDetails.getHelp();
+    assertThat(help).isNotNull();
+    assertThat(help.getLinks(0).getDescription()).isEqualTo("Help: first showcase help link");
+
+    LocalizedMessage localizedMessage = errorDetails.getLocalizedMessage();
+    assertThat(localizedMessage).isNotNull();
+    assertThat(localizedMessage.getLocale()).isEqualTo("fr-CH");
+    assertThat(localizedMessage.getMessage())
+        .isEqualTo("This LocalizedMessage should be treated specially");
+
+    // Verify custom PoetryError can be unpacked and matches expected poem
+    PoetryError poetryError = errorDetails.getMessage(PoetryError.class);
+    assertThat(poetryError).isNotNull();
+    assertThat(poetryError.getPoem()).isEqualTo(expectedPoem);
+  }
+
+  // Verifies error details are correctly propagated and unpacked over standard gRPC protocol
+  @Test
+  void testGrpc_failEchoWithDetails() {
+    FailEchoWithDetailsRequest request = FailEchoWithDetailsRequest.newBuilder().build();
+    ApiException exception =
+        assertThrows(ApiException.class, () -> grpcClient.failEchoWithDetails(request));
+
+    assertThat(exception.getStatusCode().getCode()).isEqualTo(StatusCode.Code.ABORTED);
+    assertThat(exception.getErrorDetails()).isNotNull();
+
+    // Reuse Transport-neutral Validation
+    verifyErrorDetailsContent(exception.getErrorDetails(), "roses are red");
+  }
+
+  // Verifies custom Error Details messages reflect user-defined inputs via gRPC
+  @Test
+  void testGrpc_failEchoWithDetails_customMessage() {
+    String customMessage = "this is a custom message to echo back";
+    FailEchoWithDetailsRequest request =
+        FailEchoWithDetailsRequest.newBuilder().setMessage(customMessage).build();
+    ApiException exception =
+        assertThrows(ApiException.class, () -> grpcClient.failEchoWithDetails(request));
+
+    assertThat(exception.getStatusCode().getCode()).isEqualTo(StatusCode.Code.ABORTED);
+    assertThat(exception.getErrorDetails()).isNotNull();
+
+    PoetryError poetryError = exception.getErrorDetails().getMessage(PoetryError.class);
+    assertThat(poetryError).isNotNull();
+    assertThat(poetryError.getPoem()).isEqualTo(customMessage);
+  }
+
+  // Verifies error details are accessible in raw form over REST/HTTP and validates manually-parsed
+  // decompression
+  @Test
+  void testHttpJson_failEchoWithDetails() throws Exception {
+    FailEchoWithDetailsRequest request = FailEchoWithDetailsRequest.newBuilder().build();
+    ApiException exception =
+        assertThrows(ApiException.class, () -> httpjsonClient.failEchoWithDetails(request));
+    assertThat(exception.getStatusCode().getCode()).isEqualTo(StatusCode.Code.ABORTED);
+
+    // GAX HTTP/JSON parser limitation: Because the response contains a custom/unregistered type
+    // (PoetryError) in the Any details list, HttpJsonErrorParser fails to parse the status payload,
+    // resulting in empty ErrorDetails (where getErrorInfo() returns null).
+    // Note: Standard Google Cloud services follow AIP-193 error details (ErrorInfo, RetryInfo,
+    // etc.)
+    // and do not use custom error payload types like PoetryError.
+    ErrorDetails errorDetails = exception.getErrorDetails();
+    if (errorDetails != null) {
+      assertThat(errorDetails.getErrorInfo()).isNull();
+    }
+
+    // Workaround REST limitation: Parse the raw HTTP JSON error response manually using a custom
+    // TypeRegistry that registers standard types plus the showcase-specific PoetryError type.
+    // This mimics the behavior of HttpJsonErrorParser, which currently does not support passing in
+    // a custom TypeRegistry.
+    assertThat(exception.getCause()).isInstanceOf(HttpResponseException.class);
+    HttpResponseException httpException = (HttpResponseException) exception.getCause();
+    String errorJson = httpException.getContent();
+    assertThat(errorJson).isNotNull();
+
+    JsonFormat.Parser jsonParser =
+        JsonFormat.parser().ignoringUnknownFields().usingTypeRegistry(SHOWCASE_ERROR_TYPE_REGISTRY);
+
+    // Parse the raw HTTP error JSON payload formatted per AIP-193 HTTP/1.1 representation
+    // (rest_error.proto),
+    // where fields are wrapped under the top-level "error" JSON key.
+    JsonObject root = JsonParser.parseString(errorJson).getAsJsonObject();
+    JsonObject errorObj = root.getAsJsonObject("error");
+    Status.Builder statusBuilder = Status.newBuilder();
+    jsonParser.merge(errorObj.toString(), statusBuilder);
+    Status status = statusBuilder.build();
+
+    // Verify we can successfully unpack the details from our custom status instance
+    ErrorDetails parsedDetails =
+        ErrorDetails.builder().setRawErrorMessages(status.getDetailsList()).build();
+
+    // Reuse Transport-neutral Validation!
+    verifyErrorDetailsContent(parsedDetails, "roses are red");
+  }
+
+  // Verifies custom Error Details messages reflect user-defined inputs via HTTP/JSON
+  @Test
+  void testHttpJson_failEchoWithDetails_customMessage() throws Exception {
+    String customMessage = "this is a custom message to echo back";
+    FailEchoWithDetailsRequest request =
+        FailEchoWithDetailsRequest.newBuilder().setMessage(customMessage).build();
+    ApiException exception =
+        assertThrows(ApiException.class, () -> httpjsonClient.failEchoWithDetails(request));
+
+    assertThat(exception.getStatusCode().getCode()).isEqualTo(StatusCode.Code.ABORTED);
+    assertThat(exception.getCause()).isInstanceOf(HttpResponseException.class);
+
+    HttpResponseException httpException = (HttpResponseException) exception.getCause();
+    String errorJson = httpException.getContent();
+    assertThat(errorJson).isNotNull();
+
+    JsonFormat.Parser jsonParser =
+        JsonFormat.parser().ignoringUnknownFields().usingTypeRegistry(SHOWCASE_ERROR_TYPE_REGISTRY);
+
+    JsonObject root = JsonParser.parseString(errorJson).getAsJsonObject();
+    JsonObject errorObj = root.getAsJsonObject("error");
+    Status.Builder statusBuilder = Status.newBuilder();
+    jsonParser.merge(errorObj.toString(), statusBuilder);
+    Status status = statusBuilder.build();
+
+    ErrorDetails parsedDetails =
+        ErrorDetails.builder().setRawErrorMessages(status.getDetailsList()).build();
+    PoetryError poetryError = parsedDetails.getMessage(PoetryError.class);
+    assertThat(poetryError).isNotNull();
+    assertThat(poetryError.getPoem()).isEqualTo(customMessage);
+  }
+}