fix(lorawan): prevent permanent WOULD_BLOCK when duty-cycle backoff_time is zero

[hallard]

Summary

In LoRaMac::schedule_tx(), when set_next_channel() returns LORAWAN_STATUS_DUTYCYCLE_RESTRICTED with backoff_time == 0, the original code skipped both starting the backoff timer and setting _can_cancel_tx. However, the caller (process_scheduling_state in LoRaWANStack.cpp) still set tx_ongoing = true and transitioned to DEVICE_STATE_SENDING.

This creates an unrecoverable stuck state:

• No timer fires → on_backoff_timer_expiry() never called → handle_scheduling_failure() never called → reset_ongoing_tx() never called
• tx_ongoing stays true permanently
• All subsequent lorawan.send() calls return LORAWAN_STATUS_WOULD_BLOCK (-1001) forever
• stop_sending() cannot recover the state either, because _can_cancel_tx is false, making clear_tx_pipe() return LORAWAN_STATUS_BUSY

The backoff_time == 0 case can occur due to sub-millisecond rounding when the remaining duty-cycle time is nearly zero at the moment set_next_channel() runs.

Fix

Enforce a minimum backoff of 1ms so the timer always fires, giving on_backoff_timer_expiry() a path to retry or invoke handle_scheduling_failure() to clean up the state.

// Before
case LORAWAN_STATUS_DUTYCYCLE_RESTRICTED:
    if (backoff_time != 0) {
        tr_debug("DC enforced: Transmitting in %lu ms", backoff_time);
        _can_cancel_tx = true;
        ...
        _lora_time.start(_params.timers.backoff_timer, backoff_time);
    }
    return LORAWAN_STATUS_OK;  // returns OK even when no timer was started!

// After
case LORAWAN_STATUS_DUTYCYCLE_RESTRICTED:
    if (backoff_time == 0) {
        backoff_time = 1;  // ensure timer always fires
    }
    tr_debug("DC enforced: Transmitting in %lu ms", backoff_time);
    _can_cancel_tx = true;
    ...
    _lora_time.start(_params.timers.backoff_timer, backoff_time);
    return LORAWAN_STATUS_OK;

Test plan

• Verify normal duty-cycle restricted behaviour (non-zero backoff_time) is unchanged
• Verify that a simulated backoff_time == 0 / DUTYCYCLE_RESTRICTED case no longer leaves tx_ongoing stuck
• Run existing LoRaWAN unit tests: connectivity/lorawan/tests/

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