4a0b32d90e
Add StartAck::SpawnFailed(String) so callers can distinguish a successful start from a failed spawn. The Start command arm now sends SpawnFailed on io::Error rather than the misleading Started. handlers.rs maps the new variant to an RpcErrorCode::SpawnFailed JSON-RPC error response. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>