Address claude-review round 5 on #919:
- The "userinfo has no exp; caching for Ns only" log fired on every fresh
userinfo validation (userinfo never returns exp) — downgrade WARNING → DEBUG;
the bounded-staleness window is already documented on _validate_via_userinfo.
- Add test_introspection_timeout_falls_through_to_userinfo: drives a real
introspection timeout (httpx.TimeoutException on the POST, caught inside
_introspect_token → None) through to a successful userinfo validation,
pinning the documented error fall-through end to end.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>