[FLINK-40519][network] Deliver priority barrier to a credited remote reader while the subpartition is blocked during recovery

[1996fanrui]

What is the purpose of the change

A blocked PipelinedSubpartition still hands out priority buffers (unaligned-checkpoint barriers) in
pollBuffer(), but isDataAvailableUnsafe() and getBuffersInBacklogUnsafe() still gate on !isBlocked.
A credited remote reader decides availability through those methods, so its one-shot priority notification
is evaluated as "not available", the reader is not enqueued, and the barrier is not delivered until recovery
ends (resumeConsumption()) — the checkpoint can time out. The local channel path is already handled via
hasPendingPriorityEvent; the remote path was not.

Brief change log

• [FLINK-40519] Extract PipelinedSubpartition#isBlockedForDelivery() and use it in pollBuffer, isDataAvailableUnsafe and getBuffersInBacklogUnsafe so a queued priority barrier is delivered to a credited reader while the subpartition is blocked.

Verifying this change

This change added a unit test:

• Added a PipelinedSubpartitionTest case asserting that, while blocked with a priority barrier queued, the credited-reader availability check reports available and the barrier is pollable.

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: yes (unaligned-checkpoint barrier delivery during recovery)
• 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

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CI report:

• 8a8e85d Azure: SUCCESS

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