fix: skip release creation if immutable release already exists

[thoroc]

Problem\nThe workflow failed because release v0.4.5 already exists as an immutable release and cannot be deleted or modified.\n\n## Solution\nInstead of trying to delete the existing release (which fails for immutable releases), we now check if the release exists and gracefully skip creation with a warning:\n\nbash\nif gh release view "${TAG}" &>/dev/null; then\n echo "Release ${TAG} already exists (immutable). Skipping release creation."\n echo "Note: Artifacts were built but cannot be uploaded to existing immutable release."\n exit 0\nfi\n\n\n## Note\nThe v0.4.5 release exists from before our immutable release fix. Future releases will work correctly since release-please won't create them early (with skip-github-release: true). To fix v0.4.5, you would need to manually delete it from the GitHub UI and re-run the workflow.

[github-actions]

@github-copilot Please review this PR focusing on:

1. Code quality, best practices, and potential bugs
2. Security vulnerabilities and unsafe patterns
3. Performance implications
4. Documentation accuracy and completeness
5. Test coverage and edge cases

Please provide specific, actionable feedback.