[FLINK-32695] [test-utils] Add flink-test-utils-source module for Source V2 tests.

[Poorvankbhatia]

What is the purpose of the change

Added a minimal, test-only utility module for FLIP-27 (Source V2) tests to reduce boilerplate when using Source V2 in Test cases. This is being done as a part of FLINK-32695 to migrate all the legacy test sources.

While migrating legacy SourceFunction–based tests to FLIP-27 (Source V2), a recurring pattern was noticed: many tests only need a mostly no-op scaffold (e.g., a source/enumerator that does nothing) or just one component—most often the SourceReader—to exercise a specific behavior. Hence this change is being made so that those patterns can be captured and test authors can plug in a tiny override (reader/enumerator/serializer) without re-implementing boilerplate.

Module path: flink-test-utils-parent/flink-test-utils-source

Brief change log

• Add new module in flink-test-utils-parent (packaging: jar).
• Introduced a reusable scaffolding under org.apache.flink.test.util.source:
  • AbstractTestSource — default BOUNDED Source V2 with sensible no-op implementations; easy to extend.
  • TestSourceReader — minimal SourceReader (default END_OF_INPUT), plumbs SourceReaderContext.
  • TestSplit — lightweight split with singleton-style serializer for simple tests.
  • TestSplitEnumerator — assigns a single split on request and signals no-more-splits to avoid hanging tests.
  • VoidSerializer — serializer for Void enumerator checkpoints.
• Keep dependencies lean:
  • flink-core
• No production modules changed.

Verifying this change

• Module builds with the existing Flink parent
• Unit tests added in SourcePatternExamplesTest exercise the new test-utils and cover complex Source V2 behaviors

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, Javadoc added to new classes to describe intended usage and extension points.

[flinkbot]

CI report:

• b076a81 Azure: FAILURE

Bot commands

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[Poorvankbhatia]

@fapaul PTAL :)