fix(chat): reconnect silently stalled live streams

[Sg312]

Summary

When a browser SSE connection remains open but stops delivering data, chat can stay frozen on its last tool activity until refresh even while the backend continues. Bound live reads to 45 seconds without incoming chunks, then use the existing cursor-based recovery path to catch up. Heartbeat comments reset the deadline; recovery cancels only the transport reader and never resends or aborts the backend turn.

The reported staging trace confirms continued execution and successful replay on refresh. Browser socket telemetry was unavailable, so this fixes the reproduced silent-connection failure without claiming every possible display stall has the same cause.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Other

Testing

• 152 tests pass across the SSE reader, mounted chat hook, stream context, and event dispatch suites.
• The silent-read regression fails before the change. Mounted hook regressions recover both initial and tail connections after a finished tool, preserving the cursor and message with one send and no backend abort.
• Keepalive comments sustain a long tool operation; timeout cancellation does not wait for a hung underlying source.
• App typecheck, Biome, API validation audit, and git diff checks pass.
• Browser verification skipped at the user's request. Wait for completed code review and CI before staging merge.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Sep 24, 2026 9:00pm UTC

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge based on the reviewed stream and recovery paths.

Summary

The PR adds a 45-second per-read idle deadline to workspace chat SSE connections. A silent connection is cancelled so the existing cursor-based recovery can replay missed events and reconnect, while keepalive comments sustain healthy long-running streams.

• Adds reader tests for silent cancellation and keepalive activity.
• Adds mounted-hook tests for recovery from silent initial and tail connections without resending the message.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  A[Live chat SSE read] --> B{Chunk within 45 seconds?}
  B -->|Yes| C[Process data or keepalive]
  C --> A
  B -->|No| D[Cancel transport reader]
  D --> E[Fetch events after saved cursor]
  E --> F{Turn terminal?}
  F -->|Yes| G[Replay and finalize]
  F -->|No| H[Replay and attach new tail]
  H --> A

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