test: failing test for hydration race when a subscriber exists before RouterProvider mounts

[marlass]

Same scenario as the existing race-condition regression test, plus a single router.subscribe(() => {}) call before RouterProvider mounts, which is what Sentry's wrapCreateBrowserRouterV7 instrumentation does at router creation. Because bufferedInitialStateUpdate is only armed when updateState runs with zero subscribers, RouterProvider's layout-effect subscription gets no replay and the app renders the HydrateFallback forever.

The bug from #14356 is back in 7.16.0+ and 8.x, with one extra condition: a subscriber attached to the router before RouterProvider mounts. The most common real-world source is Sentry — wrapCreateBrowserRouterV7 calls router.subscribe(...) at router creation. Combined with anything suspending above RouterProvider (code-split shell, i18n bundle), fast loaders / cached route.lazy chunks resolve before RouterProvider's subscribe() layout effect runs, the initialization update is lost, and the app renders the HydrateFallback forever. Fast responses fail, slow ones work, so it presents as an intermittent hang that is worst on warm caches.

const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))

// Something suspends above RouterProvider and resolves at ~100ms
// our app use case: loading translations
const suspensePromise = sleep(100)

const router = createBrowserRouter([
  {
    path: '*',
    // Hangs when this lands ~120-300ms after mount; 800ms works
    loader: () => sleep(200).then(() => 'LOADER_DATA'),
    Component: () => <p>Data: {useLoaderData()}</p>,
    HydrateFallback: () => <p>Hydrating... (stuck here forever)</p>,
  },
])

// THE TRIGGER: what Sentry's wrapCreateBrowserRouter does at creation.
// Comment this line out and the app renders fine.
router.subscribe(() => {})

function App() {
  use(suspensePromise)
  return <RouterProvider router={router} />
}

createRoot(document.getElementById('root')).render(
  <Suspense fallback={<p>Loading...</p>}>
    <App />
  </Suspense>
)