fix(gateway): guard _agent_cache_lock access in reset handler

Use getattr guard for _agent_cache_lock in _handle_reset_command
because test fixtures may create GatewayRunner without calling
__init__, leaving the attribute unset.

Fixes e2e test failure: test_new_resets_session,
test_new_then_status_reflects_reset, test_new_is_idempotent.

gateway/run.py

@@ -3364,15 +3364,18 @@ class GatewayRunner:
             logger.debug("Gateway memory flush on reset failed: %s", e)
         # Close tool resources on the old agent (terminal sandboxes, browser
         # daemons, background processes) before evicting from cache.
-        with self._agent_cache_lock:
-            _cached = self._agent_cache.get(session_key)
-            _old_agent = _cached[0] if isinstance(_cached, tuple) else _cached if _cached else None
-        if _old_agent is not None:
-            try:
-                if hasattr(_old_agent, "close"):
-                    _old_agent.close()
-            except Exception:
-                pass
+        # Guard with getattr because test fixtures may skip __init__.
+        _cache_lock = getattr(self, "_agent_cache_lock", None)
+        if _cache_lock is not None:
+            with _cache_lock:
+                _cached = self._agent_cache.get(session_key)
+                _old_agent = _cached[0] if isinstance(_cached, tuple) else _cached if _cached else None
+            if _old_agent is not None:
+                try:
+                    if hasattr(_old_agent, "close"):
+                        _old_agent.close()
+                except Exception:
+                    pass
         self._evict_cached_agent(session_key)
 
         try: