[v3-1-test] Add CI workflow to validate newsfragment PR numbers (#62975)

[github-actions]

• Add CI workflow to validate newsfragment PR numbers

Newsfragment files follow the naming convention {pr_number}.{type}.rst,
but nothing currently validates that the PR number in the filename matches
the actual PR number. This has led to cases where a contributor copies a
newsfragment from another PR or makes a typo, and the mismatch goes
unnoticed until a reviewer catches it manually.

The existing scripts/ci/prek/newsfragments.py validation script runs as
a local pre-commit hook where the PR number is not yet known, so it cannot
perform this check. Rather than extending that script with optional CLI
args and a separate CI invocation, this adds a standalone lightweight
workflow that uses gh pr diff --name-only to get the list of changed
files, greps for newsfragment .rst files, and checks that none have a
mismatched PR number — all in a single piped command, no checkout needed.

Notes for reviewers:

• gh pr diff --name-only includes deleted files. In practice, newsfragment
  deletions only happen during towncrier releases on main, not in contributor
  PRs, so this is not a concern for the pull_request trigger.
• GH_TOKEN: ${{ github.token }} follows the same pattern as
  milestone-tag-assistant.yml and backport-cli.yml which also call gh
  CLI directly.
• The pull-requests: read permission is required for gh pr diff to work
  on fork PRs.

• fixup! Add CI workflow to validate newsfragment PR numbers

fixup! Add CI workflow to validate newsfragment PR numbers

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com

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(cherry picked from commit dd755d8)

Co-authored-by: Xiaodong Deng xd_deng@hotmail.com
Co-authored-by: Jarek Potiuk jarek@potiuk.com
Co-authored-by: Claude Opus 4.6 noreply@anthropic.com