feat(js): durable streaming for flows

[pavelgj]

Genkit flows can be slow operations that stream intermediate progress. In scenarios with unreliable network conditions or client-side interruptions (e.g., page reloads), clients can lose their connection to the stream. Without a durability mechanism, the stream is lost, and the client cannot retrieve the in-progress or final results.

This PR introduces the concept of a StreamManager which is responsible for persisting the state of a stream. A StreamManager can be provided when defining a flow handler.

Three StreamManager implementations are included:

• InMemoryStreamManager: Stores stream state in memory. This is useful for development and testing but is not truly durable as it will lose state if the process restarts.
• firestoreStreamManager: A durable StreamManager that uses Firebase Firestore to store stream state.
• rtdbStreamManager: A durable StreamManager that uses the Firebase Realtime Database to store stream state.

To make a flow's streams durable, pass a streamManager instance to the handler. This is supported in both the Express (expressHandler) and Next.js (appRoute) plugins. The new durable-streaming sample app demonstrates this for Express:

// From js/testapps/durable-streaming/src/index.ts

const fApp = initializeApp();
export const firestore = firestoreStreamManager({
  firebaseApp: fApp,
  db: getFirestore(fApp),
  collection: 'streamy',
});

// ...

app.post(
  '/streamyFirestore',
  expressHandler(streamy, { streamManager: firestore })
);

The sample app includes a streamyThrowy flow that intentionally throws an error mid-stream to demonstrate how a durable stream manager can preserve the stream's state up to the point of failure.

A client can then connect to a durable stream and resume it if necessary:

// Start a new stream
const result = streamFlow({
  url: `http://localhost:8080/myFlowDurable`,
  input: 'tell me a long story',
});
const streamId = await result.streamId; // Save this ID

// ... later, reconnect if needed ...
const reconnectedResult = streamFlow({
  url: `http://localhost:8080/myFlowDurable`,
  streamId: streamId,
});

Beta Feature: This is to be released as a beta feature.

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)