Reuse existing WordPress upgrade pull request

[srtfisher]

Summary

• Reuse the newest open, same-repository pull request created by the github-actions app instead of opening duplicate upgrade PRs.
• Update the reused PR title and checklist, and add one idempotent comment noting the previous and new WordPress versions.
• Repair PR metadata on retries, abort safely when GitHub lookups fail, and preserve new-PR behavior when no reusable PR exists.
• Add behavior tests and run them in the existing CI workflow.

Testing

• ./tests/check-upgrade.sh
• bash -n check-upgrade.sh tests/check-upgrade.sh tests/stubs/*
• YAML parsing for action.yml and .github/workflows/test.yml
• git diff --check 28257e0..HEAD

[mogmarsh]

👍