fix(gateway): don't treat group session user_id as thread_id in shutdown notifications (#10546)

_parse_session_key() blindly assigned parts[5] as thread_id for all
chat types. For group sessions with per-user isolation, parts[5] is
a user_id, not a thread_id. This could cause shutdown notifications
to route with incorrect thread metadata.

Only return thread_id for chat types where the 6th element is
unambiguous: dm and thread. For group/channel sessions, omit
thread_id since the suffix may be a user_id.

Based on the approach from PR #9938 by @Ruzzgar.

gateway/run.py

@@ -486,9 +486,14 @@ def _parse_session_key(session_key: str) -> "dict | None":
     """Parse a session key into its component parts.
 
     Session keys follow the format
-    ``agent:main:{platform}:{chat_type}:{chat_id}[:{thread_id}[:{user_id}]]``.
+    ``agent:main:{platform}:{chat_type}:{chat_id}[:{extra}...]``.
     Returns a dict with ``platform``, ``chat_type``, ``chat_id``, and
     optionally ``thread_id`` keys, or None if the key doesn't match.
+
+    The 6th element is only returned as ``thread_id`` for chat types where
+    it is unambiguous (``dm`` and ``thread``).  For group/channel sessions
+    the suffix may be a user_id (per-user isolation) rather than a
+    thread_id, so we leave ``thread_id`` out to avoid mis-routing.
     """
     parts = session_key.split(":")
     if len(parts) >= 5 and parts[0] == "agent" and parts[1] == "main":
@@ -497,7 +502,7 @@ def _parse_session_key(session_key: str) -> "dict | None":
             "chat_type": parts[3],
             "chat_id": parts[4],
         }
-        if len(parts) > 5:
+        if len(parts) > 5 and parts[3] in ("dm", "thread"):
             result["thread_id"] = parts[5]
         return result
     return None