Pass event-level flags through to bot handlers
Zulip puts flags like "mentioned" on the event envelope, not the
message object. Previously the handler had no access to these flags,
causing mention detection to fail for display-name mentions like
@**Poe**. Thread the flags as a string list through the handler type
so handlers can check them directly.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>