Handle invalid transitions in Commit Status POST API

What does this MR do and why?

Handle invalid transitions in Commit Status POST API

This commit implements idempotent behavior for the commit status API by handling invalid state transitions gracefully.

Key changes:

1. Skip state transitions when job is already in target state
2. Handle invalid transitions by allowing external systems to override GitLab's state machine rules when appropriate
3. Add proper error handling and logging for transition failures

This addresses the issue where posting the same status twice would cause a bad request error instead of being idempotent.

Fixes: #556757

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