Fix backport pipeline issues (#2272)
- Fix graceful exit when backport-to-stable label is missing
- Fix version parsing to handle 'v' prefixes correctly
- Add proper conditional execution to prevent unnecessary steps
- Improve error handling and validation
- Add better logging for debugging
- Fix environment variable references in PR creation step
- Enhance PR description formatting and duplicate detection
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Co-authored-by: Claude <noreply@anthropic.com>