test(proxy): handle write errors in graphql query forwarding test

Agent-Logs-Url: https://github.com/github/gh-aw-mcpg/sessions/4809eb86-df75-49ee-94fe-225e87c3467d

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

internal/proxy/handler_test.go

@@ -194,7 +194,8 @@ func TestServeHTTP_GraphQLQueryStringForwardedToUpstream(t *testing.T) {
 		receivedURL = r.URL.RequestURI()
 		w.Header().Set("Content-Type", "application/json")
 		w.WriteHeader(http.StatusOK)
-		w.Write([]byte(`{"data":{"repository":{"issues":{"nodes":[]}}}}`)) //nolint:errcheck
+		_, err := w.Write([]byte(`{"data":{"repository":{"issues":{"nodes":[]}}}}`))
+		require.NoError(t, err)
 	}))
 	defer upstream.Close()