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/CayenneLPP.h

@@ -0,0 +1,11 @@
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+
+class CayenneLPP {
+public:
+  explicit CayenneLPP(size_t) {}
+  const uint8_t* getBuffer() const { return nullptr; }
+  uint16_t getSize() const { return 0; }
+};