Fix the Edit Single File behaviour

What does this MR do and why?

Related to: Code changes lost when commit message violates ... (#487761 - closed)

When editing a file directly ("Edit single file"), if a user attempts to commit changes with a message that doesn't comply with the configured push rules, the commit is correctly rejected. However, the code changes are lost, requiring the user to re-enter their modifications.

This happens because, when updating the file fails, the response incorrectly includes a 200 OK status code, causing the frontend to treat it as a success.

This MR resolves the issue by sending a 422 Unprocessable Entity status code when updating a file fails, along with an appropriate error message.status code.

Local test

Screen_Recording_2025-01-31_at_7.50.22_pm

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