[FLINK-39140][checkpoint] Enhance Unaligned Checkpoint ITCases to perform checkpointing during recovery

[1996fanrui]

What is the purpose of the change

Current Unaligned Checkpoint ITCases only restart once from a normal checkpoint. They do not cover restoring from a checkpoint produced by recovery phase — which is the key scenario for checkpointing during recovery.

Proposed mechanism: After restoring from a checkpoint, wait for the first new checkpoint to be produced, then immediately trigger a restart from it. Repeat for a configurable number of rounds (≥ 2). Whether to rescale depends on the specific test case.

This mechanism works on the current master (validating normal checkpoint recovery). Once checkpointing during recovery is enabled, the same tests automatically cover recovery-phase checkpoint scenarios.

Brief change log

• [FLINK-39140][checkpoint] Disable CUSTOM_PARTITIONER in unaligned checkpoint it case since it does not work well
  • See FLINK-39162
• [FLINK-39140][checkpoint] Enhance Unaligned Checkpoint ITCases to perform checkpointing during recovery

Verifying this change

This is an enhancement for existing ITCase, so it has been covered by testing.

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

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CI report:

• bb0667e Azure: SUCCESS

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