[FLINK-39620] [table-runtime] Add restored cumulative window regression

[qiuyanjun888]

What is the purpose of the change

This pull request adds regression coverage for FLINK-39620. It verifies that after restoring a cumulative window aggregate, flushing the global window buffer before receiving another watermark does not re-emit expired cumulative windows.

Brief change log

• Added a global cumulative window aggregate recovery regression test that snapshots, restores, flushes the buffer, and confirms old windows are not emitted again.
• Added a small test helper for building global window aggregate operators with local accumulator rows.

Verifying this change

This change added tests and can be verified as follows:

• ./mvnw -pl flink-table/flink-table-runtime -Dtest=SlicingWindowAggOperatorTest#testGlobalEventTimeCumulativeWindowsDoNotRefireExpiredWindowAfterRestore -DskipITs -Drat.skip=true test
• ./mvnw -pl flink-table/flink-table-runtime -Dtest=SlicingWindowAggOperatorTest -DskipITs -Drat.skip=true test

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: 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 (Hermes Agent / OpenAI GPT-5.5)

Generated-by: Hermes Agent (OpenAI GPT-5.5)

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CI report:

• b8386e5 Azure: FAILURE

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