[FLINK-32695] [Tests] Migrate CoGroupJoinITCase new Source/Sink V2

[Poorvankbhatia]

What is the purpose of the change

This pull request migrates CoGroupJoinITCase integration tests from deprecated SourceFunction/SinkFunction APIs to modern Source V2 and Sink V2 APIs.

Brief change log

• Replaced 5 inline SourceFunction implementations with env.fromData() calls for static test data
• Migrated 3 SinkFunction implementations to custom CollectingSink using Sink V2 API
• Maintained identical test behavior and assertions while modernizing the underlying APIs

Verifying this change

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

• All test methods in CoGroupJoinITCase (testCoGroup, testJoin, testSelfJoin, testCoGroupOperatorWithCheckpoint)
• Tests verify identical windowed join/coGroup behavior with updated APIs
• Same expected results and assertions maintained across the migration.

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]

Hey @afedulov PTAL. Thanks :)

[flinkbot]

CI report:

• 5b10d8b Azure: SUCCESS

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[fapaul]

Thanks for also fixing up the general test structure 👍