Add workflow to test community PRs via ok-to-test label

[skyamgarp]

Summary

• Adds a new GitHub Actions workflow (community_pr_test.yml) that triggers when a maintainer applies the ok-to-test label to a community (fork) PR
• Creates a test PR from the fork's commits as a branch on the main repo, allowing CI to run with repository secrets
• Notifies the original community PR if CI fails on the test PR

Details

• Uses pull_request_target so GITHUB_TOKEN has write access and secrets are available
• Verifies the labeler has write/maintain/admin access — removes the label and comments if not
• Rebases onto main before pushing; comments on the PR if there are conflicts
• Force-pushes the test branch on each label application to pick up new commits
• A second job (notify-on-failure) listens for CI failures on community-pr-test-* branches and comments on the original PR

Test plan

• Apply ok-to-test label as a maintainer on a fork PR — test PR should be created
• Apply label as a triage collaborator — label should be removed with a comment
• Apply label on a PR with merge conflicts — conflict comment should appear, no test PR created
• Re-apply label after new commits — test branch should be force-pushed and test PR updated
• Fail CI on a test PR — comment should appear on the original community PR

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