fix(gateway): enforce auth check in busy-session path to prevent unauthorized injection (#17775)

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

gateway/run.py

@@ -1874,6 +1874,22 @@ class GatewayRunner:
         merge_pending_message_event(adapter._pending_messages, session_key, event)
 
     async def _handle_active_session_busy_message(self, event: MessageEvent, session_key: str) -> bool:
+        # --- Authorization gate (#17775) ---
+        # The cold path (_handle_message) checks _is_user_authorized before
+        # creating a session.  The busy path must enforce the same check;
+        # otherwise unauthorized users in shared threads (Slack/Telegram/Discord)
+        # can inject messages into an active session they don't own.
+        if not self._is_user_authorized(event.source):
+            logger.warning(
+                "Dropping message from unauthorized user in active session: "
+                "user=%s (%s), platform=%s, session=%s",
+                event.source.user_id,
+                event.source.user_name,
+                event.source.platform.value if event.source.platform else "unknown",
+                session_key,
+            )
+            return True  # handled (silently dropped); do not fall through
+
         # --- Draining case (gateway restarting/stopping) ---
         if self._draining:
             adapter = self.adapters.get(event.source.platform)