Follow-up to the salvaged #80489 substring-fallback removal: the
structured 401/403 branch still exited with a bare return, leaving
_running True — dead listener, healthy-looking is_connected(), gateway
never told (the zombie half of the bug, OOF-156 class). It now sets a
non-retryable mattermost_auth_error with token guidance and notifies
the gateway fatal handler.
Also: pytest.importorskip for aiohttp in the verifier probe file
(module-level import crashed collection in envs without the optional
dep), and probe fixtures updated for the escalation attributes.
f4749a77a5