[SPARK-56557][SQL][TESTS] Use abstract SparkSession in SparkPlanTest

[zhengruifeng]

What changes were proposed in this pull request?

Remove the classic SparkSession override from SparkPlanTest, letting it inherit the abstract SparkSession from SparkSessionProvider. Also remove the now-unused classic import.

// Before
private[sql] abstract class SparkPlanTest extends SparkFunSuite with SparkSessionProvider {
  override protected def spark: classic.SparkSession
  ...
}

// After
private[sql] abstract class SparkPlanTest extends SparkFunSuite with SparkSessionProvider {
  ...
}

The existing import org.apache.spark.sql.classic.ClassicConversions._ handles implicit conversions to classic types where needed internally (e.g., when passing spark.sparkSession to new QueryExecution(...) which takes a classic SparkSession).

Why are the changes needed?

SparkPlanTest currently requires a classic.SparkSession, which tightly couples it to the classic implementation. Widening the type to the abstract SparkSession follows the same pattern as SPARK-56483 (SQLTestUtilsBase) — makes the test infrastructure more uniform and reduces direct references to the classic package in test utilities.

12 test suites extend SparkPlanTest and continue to work unchanged: SortSuite, ExchangeSuite, BroadcastExchangeSuite, ReuseExchangeAndSubquerySuite, TakeOrderedAndProjectSuite, BaseScriptTransformationSuite, InnerJoinSuite, OuterJoinSuite, ExistenceJoinSuite, SingleJoinSuite, BatchEvalPythonExecSuite, ExtractPythonUDFsSuite.

Does this PR introduce any user-facing change?

No. SparkPlanTest is private[sql].

How was this patch tested?

CI compilation across all modules.

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Claude Code (claude-opus-4-6)

[zhengruifeng]

merged to master