[FLINK-40068][tests] Do not assert apply() was called when cancelling KeyedJob window

[MartijnVisser]

What is the purpose of the change

Fixes the recurring KeyedComplexChainTest.testMigrationAndRestore failures (NoResourceAvailableException, Azure builds 76400, 76448, 76627), the follow-up investigation deferred in FLINK-39918. KeyedJob.StatefulWindowFunction.close() asserted applyCalled on every close, but close() also runs on the cancellation path, and the GENERATE/MIGRATE jobs are always stopped via non-draining cancel-with-savepoint, so under CI load a window subtask can be closed before its element was processed. The AssertionFailedError thrown from close() fails the shared static MiniCluster's only TaskManager (NUM_TMS=1, no restart), so the subsequent restore step and every later savepoint parameterization starve with NoResourceAvailableException after the slot-request timeout. There is no slot leak and no data loss: the runtime released all slots correctly, and the window's keyed state is restored from the savepoint independently of apply() having run.

Brief change log

• Restrict StatefulWindowFunction.close()'s applyCalled assertion to ExecutionMode.RESTORE, the only mode whose job runs to completion.

Verifying this change

This change is already covered by existing tests: KeyedComplexChainTest ran 3x locally, 16/16 green each time. The original failure needs cancel-with-savepoint to beat element delivery under CI load and is not reproducible locally.

Coverage trade-off: MIGRATE loses the applyCalled fail-safe, but the RESTORE run re-validates the migrated state end-to-end via the untouched apply() state-comparison assertions, so migration correctness is still verified.

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

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

• Yes (Claude Opus 4.8, via Claude Code)

Generated-by: Claude Opus 4.8 (1M context)

[flinkbot]

CI report:

• f66e43d Azure: SUCCESS

Bot commands

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• @flinkbot run azure re-run the last Azure build

[spuru9]

LGTM.
Clean and straightforward fix.