[CI] DRY fix action names in PR-action workflow (#5894)

[pratik-mahalle]

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This PR keeps the PR-action workflow DRY by dynamically validating fix action names against npm run -s _list:fix:* instead of relying on hardcoded patterns.

Benefits

• Prevents script-name mismatches like this one
• No need to manually maintain a list of valid fix actions in the workflow
• Invalid /fix:xxx commands now fail fast with a clear error message showing valid options

Fixes #5894