`BridgeMonitor` backward compatibility with `Serial.end()`

[mirinae3145]

On Arduino UNO Q with the new Serial alias, a conventional code calling Serial.end() does not compile because the BridgeMonitor class does not provide an end() method.

Example

#include <Arduino.h>
#include <Arduino_RouterBridge.h>

void setup() {
  Serial.begin(115200);
  Serial.println("hello");
  Serial.end();
}

void loop() {}

Proposed fix

Add an end() method to BridgeMonitor, for example:

void end() {
  // No-op
}

This would allow existing Arduino code and libraries that use begin() / end() pattern to compile without changing the runtime behavior of the monitor bridge.