fix: prevent repeated text on reconnect by using block snapshots

[VibhorGautam]

Summary

Fixes #938

When a client reconnects to an in-progress session (e.g. after a network blip or tab switch), the subscribe() method in SessionManager replayed every historical event — every block creation and every incremental updateBlock patch. This caused the client to visually rebuild content from scratch, producing duplicated text in the response.

Root cause

subscribe() stored all emitted events in an array and replayed the entire array to new subscribers. On initial connection this was harmless (array was empty), but on reconnect the array contained hundreds of events. The client already had the content from before the disconnect, so replaying everything doubled it.

Fix

Changed subscribe() to send block snapshots (current state via structuredClone) instead of replaying every historical event. Non-block milestone events (researchComplete, end, error) are still replayed so reconnecting clients know if the session already finished.

This means:

• Initial subscribers (new session) get an empty block map — no replay, same as before
• Reconnecting subscribers get the latest state of each block — no duplication
• If the session ended while disconnected, the client still receives the end signal

How to test

1. npm install && npm run dev, open http://localhost:3000
2. Send a search query and while the response is still streaming, open browser DevTools → Network tab
3. Find the active SSE/fetch connection to /api/chat and block it (or toggle your network off for a second, then back on) to force a reconnect through /api/reconnect
4. The response should continue from where it left off without the text appearing twice
5. Also let a response fully complete, then force a reconnect — the full response should show once, not doubled

[cubic-dev-ai]

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