[FLINK-40522][network] Deduplicate RecoveryCheckpointBarrier sentinel helpers shared by LocalInputChannel and RemoteInputChannel

[1996fanrui]

What is the purpose of the change

LocalInputChannel and RemoteInputChannel each defined a byte-identical pair of stateless static helpers
for the checkpointing-during-recovery sentinel protocol (releaseRetainedBuffers and
asRecoveryCheckpointBarrier). Keeping two copies invites silent drift. This extracts them into one shared
package-private class; it is a pure move with no behavior change.

Brief change log

• [FLINK-40522] Move releaseRetainedBuffers and asRecoveryCheckpointBarrier into a new package-private RecoveryCheckpointBarrierUtils; both channels delegate. The per-channel collectPreRecoveryBarrier is intentionally left as-is (the two walk different queue types).

Verifying this change

This change is a trivial rework / code cleanup without any test coverage (a pure move, no behavior change).

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: 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

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CI report:

• d23255e Azure: FAILURE

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