check_contributors.py does not notice a pull request whose commits are authored by someone else #1316

Where: scripts/check_contributors.py (pending, --identities), scripts/contributors-exceptions.json.

Current gap: PR #1311 was opened by ranjanasri6 but its one commit is authored by PRIYANKA2272004, who has no merged pull request of her own. The pending list and the checks only look at the PR author, so the second person was found by hand and had to be added to the exceptions file with a reason.

Suggested approach: Ask for each pending PR's commit authors (gh pr list --json number,commits) and list the logins that differ from the PR author under the PR, with the exception-file line to add if the person should stay unlisted.

Done when: --pending names a commit author who is not the PR author, covered by a test with saved data.