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/telemetry/listener.py

@@ -123,17 +123,20 @@ class TelemetryListener:
 
     @property
     def is_testing(self) -> bool:
-        from server.test_mode import _test_generator
-        return _test_generator.running
+        from server.test_mode import get_generator
+        gen = get_generator()
+        return gen is not None and gen.running
 
     def start_test_mode(self):
-        from server.test_mode import _test_generator
-        _test_generator.listener = self
-        _test_generator.start()
+        from server.test_mode import get_generator
+        gen = get_generator(self)
+        gen.start()
 
     def stop_test_mode(self):
-        from server.test_mode import _test_generator
-        _test_generator.stop()
+        from server.test_mode import get_generator
+        gen = get_generator()
+        if gen is not None and gen.running:
+            gen.stop()
 
     def _get_parser(self, key: str) -> BaseParser:
         if key not in self._parser_cache: