The busy-session handler (_handle_active_session_busy_message) bypassed the
authorization gate that the cold path enforces via _is_user_authorized(). In
shared-thread contexts (Slack threads, Telegram forum topics, Discord threads)
where thread_sessions_per_user=False (the default), all participants share one
session_key. An unauthorized user posting in the same thread as an authorized
user would hit the active-session branch, skip the auth check, and have their
text merged into _pending_messages or injected via agent.interrupt().
This commit adds the same _is_user_authorized() check at the top of the busy
handler, before any message queuing, steering, or interrupt logic. Unauthorized
messages are silently dropped (return True) with a warning log — matching the
cold-path behavior.
Affected platforms: Slack, Telegram, Discord, any adapter with shared-session
thread contexts.
Closes#17775
