Function enter_dstar_session 

async fn enter_dstar_session(
    radio: Radio<EitherTransport>,
    config: DStarGatewayConfig,
    tx: &UnboundedSender<Message>,
    cmd_rx: &mut UnboundedReceiver<RadioCommand>,
) -> Result<Radio<EitherTransport>, EnterDStarError>

Enter D-STAR gateway mode, run the event loop, and return the radio on clean exit.

Takes ownership of the Radio (consumed by DStarGateway::start), runs the D-STAR event loop, and returns the Radio after DStarGateway::stop.