[FLINK-32695] [Tests] Migrated TimestampITCase to SourceV2

[Poorvankbhatia]

What is the purpose of the change

This PR migrates all legacy SourceFunction usages in TimestampITCase to the modern FLIP-27 Source V2 API

Brief change log

-Replaced all env.addSource(...) usages with env.fromSource(...) using Source V2
-Introduced timestampSource(...) factory and unified MyTimestampSourceV2 + MyTimestampSourceInfiniteV2
-Consolidated MyTimestampSourceReader and MyTimestampSourceInfiniteReader into a single TimestampSourceReader
-Replaced MySourceSplit and MySourceSplitSerializer with singleton-style instances (SPLIT, SPLIT_SERIALIZER)
-Migrated tests like testTimestampExtractorWithDecreasingCustomWatermarkEmit and testTimestampExtractorWithLongMaxWatermarkFromSource to use either DataGeneratorSource or simplified Source V2
-Added rate limiting in DataGeneratorSource where applicable to allow watermarks to be emitted

Verifying this change

Existing tests already cover the migrated logic:
-TimestampITCase: All tests were adapted to use Source V2

Test coverage includes:
-testWatermarkPropagation, testTimestampHandling, testDisabledTimestamps
-Tests involving auto watermark interval and custom watermark generators
-Final watermark emission and out-of-order timestamp handling

All tests pass without behavioral changes.

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:

• fd4bcfe Azure: SUCCESS

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