[FLINK-40523][network] Compute tracePersist debug string only when trace logging is enabled

[1996fanrui]

What is the purpose of the change

NetworkActionsLogger.tracePersist(String, Buffer, ...) computed buffer.toDebugString(INCLUDE_HASH)
(a full buffer copy + hash + string allocation) before delegating to the trace-guarded overload, so
under backpressure this cost was paid for every persisted in-flight buffer even when TRACE was off.
This guards the computation behind isTraceEnabled().

Brief change log

• [FLINK-40523] Guard the toDebugString call in NetworkActionsLogger.tracePersist(...) with LOG.isTraceEnabled().

Verifying this change

This change is a trivial rework / code cleanup without any test coverage. It is behavior-preserving:
the delegate overload already no-ops when TRACE is off; only the redundant debug-string computation is skipped.

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): yes (removes redundant work on the checkpoint persist path when TRACE is off)
• 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:

• 9533db1 Azure: SUCCESS

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