fix: clear session-scoped model overrides during session reset

gateway/run.py

@@ -3264,6 +3264,10 @@ class GatewayRunner:
         except Exception:
             pass
 
+        # Clear any session-scoped model override so the next agent picks up
+        # the configured default instead of the previously switched model.
+        self._session_model_overrides.pop(session_key, None)
+
         # Reset the session
         new_entry = self.session_store.reset_session(session_key)