chore(release): process backports as part of creating RC

[rickeylev]

Automatically process pending backports when /create-rc is called.

If any backport fails to process (e.g. due to cherry-pick conflicts), the RC creation will abort.
Additionally, support --triggering-comment option to react with a thumbs-down emoji on the triggering comment if the command fails.

• Call ProcessBackports in CreateRc before proceeding with RC tagging.
• Refactor CreateRc.run to handle exceptions and reactions.
• Update GitHub Actions workflows to pass comment_id to release_create_rc workflow.
• Add unit tests for the new integration and reaction behavior.

[gemini-code-assist]

Code Review

This pull request updates the /create-rc command to automatically process pending backports before creating a release candidate. If any backport fails, the process aborts, and if triggered by a comment, it reacts with a thumbs-down emoji. The changes include updates to the documentation, a new changelog entry, the core implementation in create_rc.py, and comprehensive unit tests. The review feedback suggests two improvements: using getattr to safely access args.triggering_comment to prevent potential AttributeErrors in tests or custom namespaces, and printing the full traceback when catching unexpected exceptions to facilitate debugging in CI/CD environments.

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