[FLINK-39388][tests] Fix flaky DataGeneratorSourceITCase#testGatedRateLimiter

[featzhang]

What is the purpose of the change?

This PR fixes the flaky test DataGeneratorSourceITCase#testGatedRateLimiter by eliminating a race condition in the FirstCheckpointFilter inner class.

The root cause is that FirstCheckpointFilter previously stopped collecting elements inside snapshotState(). However, the checkpoint barrier can arrive at FirstCheckpointFilter before all upstream elements (emitted in the same checkpoint cycle by GatedRateLimiter) have been processed, causing the filter to prematurely discard elements and making the assertion assertThat(results).hasSize(capacityPerCheckpoint) fail non-deterministically.

Fixes: FLINK-39388

Brief change log

• DataGeneratorSourceITCase: Refactored FirstCheckpointFilter to implement CheckpointListener in addition to CheckpointedFunction. The element cutoff logic is moved from snapshotState() to notifyCheckpointComplete(), ensuring that collection stops only after the first checkpoint has fully completed and all in-flight elements have been processed downstream.

Verifying this change

This change is a test-only fix. The existing assertion assertThat(results).hasSize(capacityPerCheckpoint) continues to validate the correctness of GatedRateLimiter. The fix eliminates the non-deterministic timing window that caused intermittent failures.

• This change is a test-only fix.
• The fix has been verified by code review of the race condition.

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 the Flink SQL / Table API: No

Documentation

• Does this pull request introduce a new feature? No
• If yes, how is the feature documented? N/A

[flinkbot]

CI report:

• f198115 Azure: SUCCESS

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