Fix #1466: Resumable Streams

[JiwaniZakir]

Closes #1466

Before

When a user reloaded mid-stream, the browser called GET /api/chat?chatId=... to resume the stream. That route did not exist — the file app/(chat)/api/chat/route.ts only exported a POST handler. The missing route caused the request to fall through with a 204 No Content response, leaving the UI in a broken loading state with no stream data replayed.

After

GET /api/chat is now implemented and handles stream resumption end-to-end:

1. Returns 400 if chatId is absent from query params.
2. Returns 401 if the request is unauthenticated.
3. Returns 403 if the authenticated user does not own the chat.
4. Returns 204 if no chat, no stream context, or no recorded stream IDs are found (graceful no-op).
5. Otherwise, fetches the most recent stream ID via getStreamIdsByChatId({ chatId }), calls streamContext.resumeExistingStream(recentStreamId, emptyStream), and returns the result as a text/event-stream response with status 200.

Changes

• app/(chat)/api/chat/route.ts: Added GET export implementing the resumable stream logic described above. Added getStreamIdsByChatId to the import list from the DB layer. An emptyStream (ReadableStream that closes immediately) is passed as the fallback to resumeExistingStream so the response is well-formed even when the original stream has already ended.
• tests/e2e/api.test.ts: Added a "Resumable Streams" test suite with two unauthenticated cases — GET /api/chat with no chatId expects 400, and GET /api/chat?chatId=nonexistent without a session expects 401 — covering the input validation and auth guard paths.

Testing

E2E tests added in tests/e2e/api.test.ts verify the error paths directly against the running server:

✓ GET /api/chat returns 400 when chatId is missing
✓ GET /api/chat returns 401 when not authenticated

Manual verification: reloading the page mid-stream now replays buffered SSE events rather than rendering the broken empty-state shown in the issue screenshot.

---

This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.

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Suggested change

	const emptyStream = new ReadableStream({
	start(controller) {
	controller.close();
	},
	});
	return new Response(
	await streamContext.resumeExistingStream(recentStreamId, emptyStream),
	return new Response(
	await streamContext.resumeExistingStream(recentStreamId),

resumeExistingStream is called with a ReadableStream as the second argument where a number (skipCharacters) is expected, causing the stream to be coerced to NaN.

[JiwaniZakir]

— passing emptyStream as the second argument to resumeExistingStream is a type mismatch; that parameter is skipCharacters: number, so it gets coerced to NaN and the skip logic breaks silently. The fix is to drop the second argument entirely or pass 0 if you need to explicitly skip nothing. the call to match the bot's suggestion.