Fix flaky test tests/test_support_comments.py::test_a_ticket_mention_reaches_the_inbox_and_an_internal_one_only_operators

The merge lane saw `tests/test_support_comments.py::test_a_ticket_mention_reaches_the_inbox_and_an_internal_one_only_operators` fail and then pass on a rerun of the same tree. - Test: `tests/test_support_comments.py::test_a_ticket_mention_reaches_the_inbox_and_an_internal_one_only_operators` - Tree: `5bb646eda0f3a86c25137821c1f5d2c6ffeea920` - Lane task: 13885 - Flake record: 01a1a6f5-a5c8-71b8-a422-32e5504a2612 Make it deterministic; do not delete it unless it tests nothing.

Flake was a real MissingGreenlet bug: the mention DM leg reloaded the author's identity, still attached after run_deferring's rollback. Fixed in _detached; new test fails on every run without the fix. Quick gate green; PR #7972 queued. Memory: proposed 1 gotcha.