## Why
A lost Cloud reply stream previously required reopening the conversation, even when it was idle. Recover the displayed conversation automatically while holding queued input until its reply stream is usable.
## What changed
- Add bounded retries for transient stream failures in the displayed, idle conversation, with credential and network-policy checks. Keep explicit reopen behavior when recovery is unavailable or fails.
- Enable HTTP/2 keepalive pings for Cloud connections.
- Queue input while disconnected and resume ordinary submission after recovery. Mark recovered history as potentially incomplete so an explicit reopen reloads missed turns while preserving drafts and reconciling pending submissions.
- Store retained input per backend and apply background settings updates to the owning backend's drafts.
## Testing
Add regression coverage for queued input during disconnection and extend resume and backend-handoff tests to cover history refresh, draft preservation, and retained settings.
GitOrigin-RevId: 3d753e53d3c0d594680a90cac8a75fe33b91e219