[Flight] Add a cached 3rd-party component to the Flight fixture (#33443)

This should allow us to visualize what
https://github.com/facebook/react/pull/33438 is trying to convey.

An uncached 3rd-party component is displayed like this in the dev tools:

<img width="1072" alt="Screenshot 2025-06-05 at 12 57 32"
src="https://github.com/user-attachments/assets/d418ae23-d113-4dc9-98b8-ab426710454a"
/>

However, when the component is restored from a cache, it looks like
this:

<img width="1072" alt="Screenshot 2025-06-05 at 12 56 56"
src="https://github.com/user-attachments/assets/a0e34379-d8c0-4b14-8b54-b5c06211232b"
/>

The `Server Components ⚛` track is missing completely here, and the
`Loading profile...` phase also took way longer than without caching the
3rd-party component.

On `main`, the `Server Components ⚛` track is not missing:

<img width="1072" alt="Screenshot 2025-06-05 at 14 31 20"
src="https://github.com/user-attachments/assets/c35e405d-27ca-4b04-a34c-03bd959a7687"
/>

The cached 3rd-party component starts before the current render, and is
also not excluded here, which is of course expected without #33438.

fixtures/flight/src/App.js

@@ -1,4 +1,7 @@
 import * as React from 'react';
+import {renderToPipeableStream} from 'react-server-dom-webpack/server';
+import {createFromNodeStream} from 'react-server-dom-webpack/client';
+import {PassThrough, Readable} from 'stream';
 
 import Container from './Container.js';
 
@@ -35,8 +38,50 @@ async function Bar({children}) {
   return <div>{children}</div>;
 }
 
+async function ThirdPartyComponent() {
+  return new Promise(resolve =>
+    setTimeout(() => resolve('hello from a 3rd party'), 30)
+  );
+}
+
+// Using Web streams for tee'ing convenience here.
+let cachedThirdPartyReadableWeb;
+
+function fetchThirdParty(Component) {
+  if (cachedThirdPartyReadableWeb) {
+    const [readableWeb1, readableWeb2] = cachedThirdPartyReadableWeb.tee();
+    cachedThirdPartyReadableWeb = readableWeb1;
+
+    return createFromNodeStream(Readable.fromWeb(readableWeb2), {
+      moduleMap: {},
+      moduleLoading: {},
+    });
+  }
+
+  const stream = renderToPipeableStream(
+    <ThirdPartyComponent />,
+    {},
+    {environmentName: 'third-party'}
+  );
+
+  const readable = new PassThrough();
+  // React currently only supports piping to one stream, so we convert, tee, and
+  // convert back again.
+  // TODO: Switch to web streams without converting when #33442 has landed.
+  const [readableWeb1, readableWeb2] = Readable.toWeb(readable).tee();
+  cachedThirdPartyReadableWeb = readableWeb1;
+  const result = createFromNodeStream(Readable.fromWeb(readableWeb2), {
+    moduleMap: {},
+    moduleLoading: {},
+  });
+  stream.pipe(readable);
+
+  return result;
+}
+
 async function ServerComponent() {
   await new Promise(resolve => setTimeout(() => resolve('deferred text'), 50));
+  return await fetchThirdParty();
 }
 
 export default async function App({prerender}) {