[SPARK-56744][INFRA] Document test base class hierarchy in AGENTS.md

AGENTS.md

@@ -20,6 +20,31 @@ Spark Connect protocol is defined in proto files under `sql/connect/common/src/m
 
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+## Scala Test Base Classes
+
+When writing a new Scala test suite, pick the lowest base class that provides what the test actually needs. Spark uses the `AnyFunSuite` ScalaTest style throughout, so the bases below are the chain to choose from. Each adds capability on top of the previous:
+
+    SparkFunSuite                                                           (core)
+      <- PlanTest                                                           (sql/catalyst)
+        <- QueryTest                                                        (sql/core)
+          <- SharedSparkSession                                             (sql/core)
+
+| Test scope | Base | Notes |
+|------------|------|-------|
+| Plain JVM/Scala — no Spark SQL | `SparkFunSuite` | `core` utilities, RDD, network, util classes, etc. Adds per-test timeout, `testRetry`, `gridTest`, thread audit, fixed timezone/locale, `withTempDir`, `withLogAppender`, `checkError`. |
+| Catalyst plan tests — no `SparkSession` | `PlanTest` | Adds `comparePlans`, `normalizePlan`, `normalizeExprIds`. For analyzer / optimizer / planner rule tests. |
+| SQL/DataFrame helpers — abstract `spark` | `QueryTest` | Adds `checkAnswer`, codegen-on/off helpers. Cannot be instantiated alone — `spark` is abstract and must be supplied by a session-providing trait. |
+| SQL/DataFrame integration tests — provides a session | `SharedSparkSession` | The default for most SQL suites. Provides a shared classic `TestSparkSession`, `testImplicits`, plus `checkAnswer` from `QueryTest`. |
+
+`QueryTest` declares `spark: SparkSession` abstractly via `SparkSessionProvider`. To run a concrete suite, mix in a session-providing trait. The common providers in this repo are:
+
+| Session provider | Module / location | Use case |
+|---|---|---|
+| `SharedSparkSession` | `sql/core` | Classic in-process `SparkSession`. Default for tests under `sql/core`. |
+| `TestHiveSingleton` | `sql/hive` | Hive-backed session (`TestHive`). Used by tests under `sql/hive`. |
+
+Linearization gotcha: the first item in the `extends` clause must transitively extend a class (i.e. carry a non-`Object` superclass). The four bases above all carry the `SparkFunSuite` chain, so they can appear first. A "pure helper" trait (e.g. `*ErrorsBase`, `*Helper`) does not — if you put one first, mix in a class-bearing trait immediately after, or compilation fails with `superclass Object is not a subclass of the superclass SparkFunSuite of the mixin trait ...`. Quick check: `grep "^trait <Name>"` — if it ends in `extends DataTypeErrorsBase` or another pure trait, it does not carry the class chain.
+
 ## Build and Test
 
 Build and tests can take a long time. If the user explicitly asked to run tests, run them. Otherwise (you are running tests on your own to verify a change), first ask the user if they have more changes to make.