Add exception handler and tests for duplicate budget constraint

apps/api/src/main/java/com/upkeep/infrastructure/adapter/in/rest/common/exception/GlobalExceptionMapper.java

@@ -1,6 +1,7 @@
 package com.upkeep.infrastructure.adapter.in.rest.common.exception;
 
 import com.upkeep.domain.exception.AlreadyMemberException;
+import com.upkeep.domain.exception.BudgetAlreadyExistsException;
 import com.upkeep.domain.exception.CompanyNotFoundException;
 import com.upkeep.domain.exception.CompanySlugAlreadyExistsException;
 import com.upkeep.domain.exception.CustomerAlreadyExistsException;
@@ -151,6 +152,13 @@ private Response handleDomainException(DomainException exception, String traceId
                     ))
                     .build();
 
+            case BudgetAlreadyExistsException e -> Response
+                    .status(409)
+                    .entity(ApiResponse.error(
+                            ApiError.of("BUDGET_ALREADY_EXISTS", e.getMessage(), traceId)
+                    ))
+                    .build();
+
             default -> Response
                     .status(422)
                     .entity(ApiResponse.error(

apps/api/src/test/java/com/upkeep/application/usecase/SetCompanyBudgetUseCaseImplTest.java

@@ -6,6 +6,7 @@
 import com.upkeep.application.port.out.audit.AuditEventRepository;
 import com.upkeep.application.port.out.budget.BudgetRepository;
 import com.upkeep.application.port.out.membership.MembershipRepository;
+import com.upkeep.domain.exception.BudgetAlreadyExistsException;
 import com.upkeep.domain.exception.MembershipNotFoundException;
 import com.upkeep.domain.exception.UnauthorizedOperationException;
 import com.upkeep.domain.model.audit.AuditEvent;
@@ -19,6 +20,7 @@
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 
+import java.time.Instant;
 import java.util.Optional;
 import java.util.UUID;
 
@@ -137,6 +139,33 @@ void shouldSetBudgetWithDifferentCurrencies() {
         assertEquals(100000L, result.amountCents());
     }
 
+    @Test
+    @DisplayName("should throw exception when budget already exists for current month")
+    void shouldThrowExceptionWhenBudgetAlreadyExists() {
+        String companyId = UUID.randomUUID().toString();
+        String userId = UUID.randomUUID().toString();
+
+        Membership ownerMembership = createOwnerMembership(userId, companyId);
+        when(membershipRepository.findByCustomerIdAndCompanyId(
+            any(CustomerId.class),
+            any(CompanyId.class)
+        )).thenReturn(Optional.of(ownerMembership));
+
+        // Mock that a budget already exists for the current month
+        Budget existingBudget = mock(Budget.class);
+        when(budgetRepository.findByCompanyIdAndEffectiveFrom(
+            any(CompanyId.class),
+            any(Instant.class)
+        )).thenReturn(Optional.of(existingBudget));
+
+        SetBudgetCommand command = new SetBudgetCommand(companyId, userId, 50000L, Currency.EUR);
+
+        assertThrows(BudgetAlreadyExistsException.class, () -> useCase.execute(command));
+
+        verify(budgetRepository, never()).save(any(Budget.class));
+        verify(auditEventRepository, never()).save(any(AuditEvent.class));
+    }
+
     private Membership createOwnerMembership(String userId, String companyId) {
         return Membership.create(
             CustomerId.from(userId),

apps/api/src/test/java/com/upkeep/infrastructure/adapter/in/rest/budget/BudgetResourceTest.java

@@ -313,4 +313,41 @@ void shouldSetBudgetWithDifferentCurrencies() {
                     .body("data.currency", equalTo(currency));
         }
     }
+
+    @Test
+    @DisplayName("should reject duplicate budget for same month")
+    void shouldRejectDuplicateBudgetForSameMonth() {
+        String uniqueId = UUID.randomUUID().toString().substring(0, 8);
+        String email = "duplicate-budget-" + uniqueId + "@example.com";
+        String token = createUserAndGetToken(email);
+        String companyId = createCompany(token, "Duplicate Budget Company", "duplicate-budget-" + uniqueId);
+
+        String requestBody = """
+                {
+                    "amountCents": 50000,
+                    "currency": "EUR"
+                }
+                """;
+
+        // First budget should succeed
+        given()
+                .contentType(ContentType.JSON)
+                .cookie("access_token", token)
+                .body(requestBody)
+                .when()
+                .post("/api/companies/" + companyId + "/budget")
+                .then()
+                .statusCode(201);
+
+        // Second budget for the same month should fail
+        given()
+                .contentType(ContentType.JSON)
+                .cookie("access_token", token)
+                .body(requestBody)
+                .when()
+                .post("/api/companies/" + companyId + "/budget")
+                .then()
+                .statusCode(409)
+                .body("error.code", equalTo("BUDGET_ALREADY_EXISTS"));
+    }
 }