Instructor v2 special-exam allowance DELETE ignores numeric user_ids that its API spec allows #39228

DELETE /api/instructor/v2/courses/<key>/special_exams/<exam_id>/allowance doesn't accept numeric user ids, although its API spec says it does.

• lms/djangoapps/instructor/docs/references/instructor-v2-special-exams-api-spec.yaml, ExamAllowanceDeleteRequest.user_ids: "List of usernames, emails, or numeric user IDs".
• The view (api_v2.py, delete(self, request, course_id, exam_id)) resolves each entry with get_user_by_username_or_email(str(user_identifier)). A numeric id is never found, and the row reports {"identifier": 10219, "success": false, "error": "User not found"}. The response is still 200.

The instructor-dashboard MFE sends exactly that: its Delete confirmation posts {"exam_id": 2, "user_ids": [10219], "allowance_type": "additional_time_granted"}. So deleting an allowance from the MFE never removes it, and it's still listed by GET …/special_exams/allowances afterwards. Add and Edit send usernames and work.

Expected: honor the documented contract by resolving numeric identifiers as user ids, for example by trying User.objects.get(id=…) for integer values. Otherwise, change the spec and reject numeric ids with a 400. Answering 200 when every row failed also hides the problem from clients; a 4xx (or 207) when nothing was deleted would surface it.

Environment: Tutor main with ENABLE_SPECIAL_EXAMS, a timed exam, 2026-09-24. Code as on master 2026-10-13. Found by the Open edX end-to-end test suite (BTR TC-00541).