Function run_aprs_loop 

async fn run_aprs_loop(
    client: &mut AprsClient<EitherTransport>,
    tx: &UnboundedSender<Message>,
    cmd_rx: &mut UnboundedReceiver<RadioCommand>,
) -> Result<(), String>

Run the APRS event loop until ExitAprs is received or a transport error occurs.

Returns Ok(()) on clean exit, Err(msg) on transport failure.