[FLINK-32695] [Tests] CoordinatorEventsToStreamOperatorRecipientExactlyOnceITCase Migrated to Source V2

[Poorvankbhatia]

What is the purpose of the change

This PR migrates ManuallyClosedSourceFunction in CoordinatorEventsToStreamOperatorRecipientExactlyOnceITCase from the deprecated legacy SourceFunction API to the modern FLIP-27 Source API

Brief change log

• Removed ManuallyClosedSourceFunction class that implemented the deprecated SourceFunction interface
• Note: DataGeneratorSource cannot be used directly as this source emits no data and requires external control via static flags for test coordination

Verifying this change

Existing tests in CoordinatorEventsToStreamOperatorRecipientExactlyOnceITCase already cover this change:

• testCheckpoint()
• testUnalignedCheckpoint()
• testCheckpointWithSubtaskFailure()
• testUnalignedCheckpointWithSubtaskFailure()

All tests pass.

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

[Poorvankbhatia]

@afedulov PTAL :)

[flinkbot]

CI report:

• fca7c71 Azure: SUCCESS

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