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/Stream.h

@@ -1,10 +1,23 @@
 #pragma once
 
-// Mock Stream class for native testing
-// Provides minimal interface needed by Utils.h
+#include <cstddef>
+#include <cstdint>
 
 class Stream {
 public:
-    virtual void print(char c) {}
-    virtual void print(const char* str) {}
+    virtual ~Stream() = default;
+    virtual size_t write(uint8_t) { return 1; }
+    virtual size_t write(const uint8_t* buffer, size_t size) {
+        size_t total = 0;
+        for (size_t i = 0; i < size; i++) {
+            total += write(buffer[i]);
+        }
+        return total;
+    }
+    virtual int available() { return 0; }
+    virtual int availableForWrite() { return 0; }
+    virtual int read() { return -1; }
+    virtual void flush() {}
+    virtual void print(char) {}
+    virtual void print(const char*) {}
 };