NIFI-15668 - Fail fast when RUN_ONCE is issued on an invalid processor

[pvillard31]

Summary

NIFI-15668 - Fail fast when RUN_ONCE is issued on an invalid processor

When a RUN_ONCE command is issued on an invalid processor, the processor enters the initiateStart() retry loop which retries every 500ms waiting for the processor to become valid. Unlike a regular RUNNING start where this retry behavior is desirable, RUN_ONCE semantically means "execute right now" — if the processor can't execute, it should fail immediately.

The retry loop causes the processor to get permanently stuck in the RUN_ONCE physical state because:

• isRunning() returns true for RUN_ONCE, so configuration and parameter changes are rejected with "Cannot modify configuration while the Processor is running"
• getScheduledState() returns RUN_ONCE (unlike STARTING which maps to STOPPED when invalid), so the API's stop handler doesn't recognize it needs to stop the processor
• The stop callback that would normally fire after execution is never invoked because the processor never reaches the trigger() call

The fix adds an early exit in StandardProcessorNode.initiateStart(): when the processor is not valid and the desired state is RUN_ONCE, it logs a warning with the validation errors and immediately returns the processor to STOPPED via completeStopAction().

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