Fix GraphQL status code never being applied to response

graphql_response() computed status_code (200 or 400) but only returned
it in an unused tuple instead of setting resp.status_code. All GraphQL
error responses were returning 200.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

responder/ext/graphql/__init__.py

@@ -101,8 +101,7 @@ class GraphQLView:
             response_data["data"] = result.data
 
         resp.media = response_data
-        status_code = 200 if not result.errors else 400
-        return (query, json.dumps(response_data), status_code)
+        resp.status_code = 200 if not result.errors else 400
 
     async def on_request(self, req, resp):
         await self.graphql_response(req, resp)