[FLINK-40086][runtime] Adds error handling to newly added preCompletionAction

[XComp]

What is the purpose of the change

FLINK-40076 introduced a slight refactoring to Execution where the accumulators and ioMetrics are updated before completing the Execution. This introduced a slight inconsistency, where (theoretically) the preCompletionAction can fail and leave the Execution in an inconsistent state with the state being terminal but the terminalFuture not being completed.

This PR fixes the inconsistency. That said - the issue doesn't exist in the current code base because the only existing preCompletionAction only consists of field setters.

Brief change log

• Adds a try/catch around the preCompletionAction execution

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

• Yes (please specify the tool below)

Issue was identified by claude. Code change was done manually.

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CI report:

• 162a021 Azure: FAILURE

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