[FLINK-39845][tests] Fix SavepointITCase.testStopWithSavepointFailsOverToSavepoint under AdaptiveScheduler

[MartijnVisser]

What is the purpose of the change

Backport of #28311 (commit c3ec569) to release-2.3.

The JUnit5 migration (FLINK-39124, #27667 — also present on release-2.3) replaced a cause-chain search in SavepointITCase.testStopWithSavepointFailsOverToSavepoint (ExceptionUtils.assertThrowable -> findThrowable) with a direct-cause assertion (assertThatThrownBy(...).hasCauseInstanceOf(StopWithSavepointStoppingException.class)).

Under the AdaptiveScheduler, StopWithSavepoint.onLeave() wraps the expected StopWithSavepointStoppingException inside a FlinkException ("Stop with savepoint operation could not be completed."), so it is no longer the direct cause. The default scheduler still exposes it directly, so the test passes there but fails under the adaptive scheduler.

This restores the cause-chain search so the test passes under both schedulers.

Brief change log

• Replace the direct-cause assertion in testStopWithSavepointFailsOverToSavepoint with FlinkAssertions.anyCauseMatches(StopWithSavepointStoppingException.class, "A savepoint has been created at: "), which searches the whole cause chain (restoring the pre-migration semantics).

(Note: the shared anyCauseMatches helper matches the message with contains rather than the original startsWith. This is safe here because the phrase is the leading text of StopWithSavepointStoppingException's own message, and anyCauseMatches binds the class and the message to the same throwable in the chain.)

Verifying this change

This change is already covered by the existing SavepointITCase.testStopWithSavepointFailsOverToSavepoint. The cherry-pick applied cleanly onto release-2.3 (which carries the same #27667 migration commit), and FlinkAssertions.anyCauseMatches(Class, String) is present on the branch.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no (test-only change)
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Was generative AI tooling used to co-author this PR?

• Yes (Claude Code (Claude Opus 4.8))

Generated-by: Claude Code (Claude Opus 4.8)

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CI report:

• 0fe56ec Azure: SUCCESS

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