Spark 4.2: UnionCodegenSuite partitioning-aware test inspects UnionExec that Comet replaces

[andygrove]

Spark 4.2's UnionCodegenSuite (SPARK-56482) test "partitioning-aware union falls back to non-codegen" inspects Spark's UnionExec whole-stage-codegen internals via executedPlan.collectFirst { case u: UnionExec => u }.get.

Comet replaces UnionExec with CometUnionExec, so collectFirst returns None and the test fails with NoSuchElementException: None.get.

Currently marked IgnoreComet in dev/diffs/4.2.0.diff (WholeStageCodegenSuite is already IgnoreCometSuite).