[FLINK-33186][checkpoint/tests] Fix flakiness in CheckpointAfterAllTasksFinishedITCase.testRestoreAfterSomeTasksFinished

[Efrat19]

What is the purpose of the change

I traced the flakiness to task finished after the checkpoint plan was calculated but before triggerCheckpoint. The task is already removed from TaskExecutor#taskSlotTable So TaskExecutor#triggerCheckpoint fails the checkpoint with error 'TaskManager received a checkpoint request for unknown task <>'.

Production systems are expected to tolerate an occasional checkpoint failure So we can address the flakiness without changes this known limitation for tasks finished mid checkpoint.

the test provisions 2 sources passA that finishes quickly and passB that blocks forever and non-deterministically waits for at least one of passA subtasks to finish, which introduces the race.
The fix here makes CheckpointAfterAllTasksFinishedITCase.testRestoreAfterSomeTasksFinished wait for all subtasks of passA to finish before triggering savepoint, to avoid the race but still achieve the test goal with passB subtasks still blocking.

Brief change log

Make CheckpointAfterAllTasksFinishedITCase.testRestoreAfterSomeTasksFinished wait for all subtasks of passA to finish before triggering savepoint, to avoid the race but still achieve the test goal with passB subtasks still blocking.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

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

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Co-Generated-by: cloud-sonnet-5

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CI report:

• 6ec03c9 Azure: SUCCESS

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