## Why
Gateway sign-in needs a way to complete authentication over SSH or when no local browser is available.
## What changed
- Prefer device authentication for gateways that support it in SSH sessions, headless Linux environments, or when no browser handler is available.
- Check the selected session's provider and complete required gateway sign-in before requesting models, including when resuming a thread.
- Display the verification URL, one-time code, and cancellation instructions. Handle sign-in results, cancellation, and connection failures while the prompt is open.
- Skip device login for older servers that do not support it while preserving errors from current servers.
## Testing
Add protocol tests for early sign-in completion, connection loss, provider selection, resumed threads, and legacy-server compatibility, plus snapshots of the device prompt and gateway sign-in loading text.
GitOrigin-RevId: 4f3675ab2fc7fccafbad17796c72a7736ebb86b5
smurching-oai
45d1f24d79