fix(gateway): restore Telegram DM topic thread_id after session split (#27166)

When context compression triggers a mid-turn session split, source.thread_id
can be None on synthetic/recovered events. _thread_metadata_for_source then
returns None, causing the Telegram adapter to send with no message_thread_id
and the response lands in the General thread instead of the active DM topic.

Fix:
- hermes_state.py: Add get_telegram_topic_binding_by_session() for reverse
  lookup by session_id (enabled by the existing UNIQUE INDEX on session_id).
- gateway/run.py: After session-split detection, if source is a Telegram DM
  and source.thread_id is None, recover it from the binding via the new
  method so _thread_metadata_for_source produces the correct thread routing.
- tests/: Coverage for the new lookup method and the recovery flow.

hermes_state.py

@@ -2852,6 +2852,30 @@ class SessionDB:
                 return []
         return [dict(row) for row in rows]
 
+    def get_telegram_topic_binding_by_session(
+        self,
+        *,
+        session_id: str,
+    ) -> Optional[Dict[str, Any]]:
+        """Return the Telegram DM topic binding for a given session_id, if present.
+
+        Uses the UNIQUE INDEX on telegram_dm_topic_bindings(session_id) for an
+        efficient reverse lookup. Returns None when the session has no binding or
+        the table does not exist yet.
+        """
+        with self._lock:
+            try:
+                row = self._conn.execute(
+                    """
+                    SELECT * FROM telegram_dm_topic_bindings
+                    WHERE session_id = ?
+                    """,
+                    (str(session_id),),
+                ).fetchone()
+            except sqlite3.OperationalError:
+                return None
+        return dict(row) if row else None
+
     def bind_telegram_topic(
         self,
         *,