fix: add pragma no cover for unused test helper callback

The _list_tools callback is required to construct the Server but is not
actually invoked during the notification test.  Marking with pragma: no cover
fixes the 99.99% -> 100% coverage gate.

Author: g97iulio1609

tests/client/test_list_changed_callbacks.py

@@ -23,7 +23,7 @@ async def on_tools_changed() -> None:
         callback_called.set()
 
     async def _list_tools(_ctx: object, _params: object) -> types.ListToolsResult:
-        return types.ListToolsResult(tools=[])
+        return types.ListToolsResult(tools=[])  # pragma: no cover
 
     server = Server(
         name="ListChangedServer",