[FLINK-32695] [Tests] Migrate ProcessingTimeServiceSource from Source Function to Source V2 API in StreamSourceOperatorLatencyMetricsTest

[Poorvankbhatia]

What is the purpose of the change

This pull request migrates the ProcessingTimeServiceSource test utility from the deprecated SourceFunction API to the new Source V2 API in StreamSourceOperatorLatencyMetricsTest.

Brief change log

• Replaced ProcessingTimeServiceSource extends SourceFunction with ProcessingTimeServiceSource implements Source<Long, SourceSplit, Void>
• Implemented required Source V2 interfaces: SourceReader, SplitEnumerator, and serializers
• Updated test execution logic to use operator.emitNext() pull-based model instead of the push-based operator.run()
• Added JavaDoc documentation clarifying the test-only nature of ProcessingTimeServiceSource

Verifying this change

This change is already covered by existing tests, such as:

• All 5 existing test cases in StreamSourceOperatorLatencyMetricsTest continue to pass:
  • testLatencyMarkEmissionDisabled
  • testLatencyMarkEmissionEnabledViaExecutionConfig
  • testLatencyMarkEmissionEnabledViaFlinkConfig
  • testLatencyMarkEmissionEnabledOverrideViaExecutionConfig
  • testLatencyMarkEmissionDisabledOverrideViaExecutionConfig

The tests verify that latency markers are correctly emitted (or not emitted) based on various configuration settings.

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

[flinkbot]

CI report:

• fd35008 Azure: SUCCESS

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[Poorvankbhatia]

@afedulov, please have a look. Thanks.