[FLINK-40077][table-planner] Fix lost NoMoreSplitsEvent masking failure cause in CommonExecSinkITCase

[MartijnVisser]

What is the purpose of the change

CommonExecSinkITCase.testNullEnforcer is flaky on CI (observed on the AdaptiveScheduler nightly leg, Azure build 76751). The test asserts that the job failure cause chain contains the NOT NULL enforcer message, but intermittently the reported terminal cause is instead FlinkException: An OperatorEvent ... was lost ... Event: '[NoMoreSplitEvent]'.

Root cause: the Source V2 test source emits all rows on the first pollNext, before the coordinator's NoMoreSplitsEvent is delivered. The NOT NULL enforcer then fails the chained Source -> ConstraintEnforcer -> Writer task; the still-in-flight NoMoreSplitsEvent reaches an already-FAILED task and, since it is not loss-tolerant, escalates to a lost-OperatorEvent failover. Under NoRestartBackoffTimeStrategy that failover wins as the reported cause and masks the enforcer error. This race is distinct from FLINK-40011 (which addressed the reader reaching FINISHED before the event arrived); it occurs on the FAILED-task path, which gating END_OF_INPUT alone cannot cover.

The underlying runtime behavior (a lost, non-loss-tolerant coordinator event to an already-failing task superseding the real application failure cause) is a pre-existing general issue tracked separately as FLINK-40078; this PR only removes the race from the test scaffolding.

Brief change log

• Gate record emission in the test source's pollNext on noMoreSplits, so both coordinator events are delivered before any record is emitted (and thus before the enforcer can fail the task), leaving no coordinator event in flight at failure time. This extends the noMoreSplits gating added in FLINK-40011 from completion to emission.

Verifying this change

This change is a test-stability fix, covered by the existing CommonExecSinkITCase tests.

• Ran CommonExecSinkITCase#testNullEnforcer 30 times under -Penable-adaptive-scheduler (the failing profile): all green.
• The other tests in the class that reuse the same source (testStreamRecordTimestampInserter*, testUnifiedSinksAreUsableWithDataStreamSinkProvider, testCharLengthEnforcer, testBinaryLengthEnforcer) still pass, since the enumerator always signals no-more-splits and the rows are still emitted.

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
• 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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CI report:

• 77df62a Azure: SUCCESS

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