## What changed
Expose `cloud_backend::connect` to adapt an authenticated `codex-cloud-client` client to `AppServerClient`. Support account reads, resuming a conversation by ID with saved settings, reading its metadata, listing the loaded thread, and unsubscribing.
Forward turn lifecycle events and agent message deltas, and track thread status from attachment notifications. Keep the previous attachment when a replacement fails, report stream disconnections, and reject turns without an active attachment. Unsupported requests and approval replies return explicit errors.
## Testing
Add tests covering failed attachment replacement, turn admission after disconnect, and thread status tracking across notifications and reattachment.
GitOrigin-RevId: 5a131e814d588cadeb9fb94513b99e92b47e3da4