Function serve 

pub async fn serve(
    cfg: ResolverConfig,
    addr: SocketAddr,
    bind_tx: Sender<Result<(), String>>,
    cancel: CancellationToken,
) -> Result<()>

Run the responder on addr over both UDP and TCP until cancel fires.

bind_tx reports the bind result so the supervisor can surface a port conflict immediately instead of discovering it from a log line.