Function handle_server_msg 

async fn handle_server_msg(
    msg: ServerMsg,
    token: &str,
    http: &Client,
    agents: &Arc<Mutex<HashSet<String>>>,
) -> Result<(), String>

Handle a message from the cloud server. The WS wake signal carries zero metadata — the sidecar polls all its registered agents via REST to find pending injections, so a compromised subscriber gains nothing.