Function enter_aprs_session 

async fn enter_aprs_session(
    radio: Radio<EitherTransport>,
    config: AprsClientConfig,
    tx: &UnboundedSender<Message>,
    cmd_rx: &mut UnboundedReceiver<RadioCommand>,
) -> Result<Radio<EitherTransport>, EnterAprsError>

Enter APRS mode, run the event loop, and return the radio on clean exit.

Takes ownership of the Radio (consumed by AprsClient::start), runs the APRS event loop, and returns the Radio after AprsClient::stop.