[FLINK-40237][cep] Clean up expired NFA state in processing-time timers

[Dream95]

What is the purpose of the change

(For example: This pull request makes task deployment go through the blob server, rather than through RPC. That way we avoid re-transferring them on each deployment (during recovery).)

This is a follow-up to FLINK-32701. CepOperator already cleans up dangling NFA state in onEventTime(), but the equivalent cleanup was missing from onProcessingTime(). As a result, after patterns expire under processing time, computationStates could retain only the initial partial match and never be cleared. This PR applies the same cleanup logic in the processing-time timer path.

Brief change log

• Clear computationStates in onProcessingTime() when only the initial partial match remains and there are no completed matches
• Add @VisibleForTesting helper hasNonEmptyNFAState(KEY) to inspect per-key NFA state
• Add testCEPOperatorClearsExpiredNFAStateInProcessingTime to verify expired NFA state is removed after the processing-time timer fires

Verifying this change

• Added CEPOperatorTest#testCEPOperatorClearsExpiredNFAStateInProcessingTime, which processes a start event under processing time, asserts NFA state is present, advances processing time past the pattern timeout, and asserts NFA state is cleared

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)

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CI report:

• b30ffd1 Azure: SUCCESS

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