fix: game plugin cache + test mode NullPointer crash on shutdown

- game_manager: add _scanned cache, _ensure_scanned() avoids re-scanning on every API call
- test_mode: fix AttributeError when stop_test_mode called before start
- listener: safe stop_test_mode with null check

server/game_manager.py

@@ -68,11 +68,11 @@ class GamePluginManager:
         USER_DIR.mkdir(parents=True, exist_ok=True)
         self._plugins: dict[str, GamePlugin] = {}
         self._parser_cache: dict[str, Any] = {}
+        self._scanned = False
         self._discover()
 
     def _discover(self):
         self._plugins.clear()
-
         for d in [BUILTIN_DIR, USER_DIR]:
             if not d.exists():
                 continue
@@ -84,14 +84,18 @@ class GamePluginManager:
                         gp = GamePlugin.from_manifest(manifest, is_builtin)
                         if gp:
                             self._plugins[gp.id] = gp
-                            logger.debug("Discovered game plugin: %s", gp.id)
+        self._scanned = True
+
+    def _ensure_scanned(self):
+        if not self._scanned:
+            self._discover()
 
     def list_all(self) -> list[dict[str, Any]]:
-        self._discover()
+        self._ensure_scanned()
         return [p.to_dict() for p in self._plugins.values()]
 
     def get(self, plugin_id: str) -> GamePlugin | None:
-        self._discover()
+        self._ensure_scanned()
         return self._plugins.get(plugin_id)
 
     def get_parser(self, plugin_id: str) -> Any | None: