## What changed
- Add `EnvironmentManager::from_accepted_websocket` so embedding hosts can
construct a remote environment from an already accepted and authenticated
Axum WebSocket.
- Add `replace_accepted_websocket` to retire the current transport and resume
the same exec-server session on a host-supplied replacement connection.
- Serialize replacement handoffs, reject overlapping replacements, and release
the handoff claim when a replacement attempt is cancelled or fails.
## Testing
- Cover initial connection validation and immediate environment readiness.
- Verify replacement retry behavior and recovery of a running process and its
output after reconnecting.
GitOrigin-RevId: 1f2ab7bcf7b5abbbece5c101801432dc84a8058d
9e680a52e7