fix: prevent ESP32 KISS modem stalls under USB backpressure

Switch ESP32-S3 KISS modem environments to HWCDC and move outbound KISS writes to a non-blocking queued frame path so loop() and TX completion keep progressing when the host reads slowly. Add native backpressure regression tests and correct the native_kiss_modem test filter so this suite runs directly with pio test.

test/mocks/helpers/SensorManager.h

@@ -0,0 +1,10 @@
+#pragma once
+
+#include <cstdint>
+#include "CayenneLPP.h"
+
+class SensorManager {
+public:
+  virtual ~SensorManager() = default;
+  virtual bool querySensors(uint8_t, CayenneLPP&) { return false; }
+};