feat(ch934x_serial): 添加串口写入功能的端口索引参数

- write方法新增可选的serialPortIndex参数，支持直接写入指定端口
- 传入端口索引时不会改变当前活跃端口，保持原有活跃端口行为
- 添加对负数端口索引的验证处理，直接返回零避免异常
- 更新文档说明多端口并发客户端应使用显式端口写入方式
- 增加相关单元测试覆盖新参数的各种使用场景
- 同步更新Android原生层实现以支持端口索引传递

test/ch934x_serial_test.dart

@@ -45,6 +45,7 @@ class _MockCh934xSerialPlatform extends Ch934xSerialPlatform
   bool closeResult;
   Uint8List? readBytes;
   int writeResult;
+  int? lastWritePort;
   bool gpioOutputResult;
   int gpioInput;
   bool modemControlResult;
@@ -63,8 +64,7 @@ class _MockCh934xSerialPlatform extends Ch934xSerialPlatform
   Future<List<Ch934xSerialPortInfo>> getSerialPortList(
     int deviceId, {
     required int interfaceNumber,
-  }) async =>
-      ports;
+  }) async => ports;
 
   @override
   Future<bool> openPort(Ch934xPortTarget target) async => openResult;
@@ -79,17 +79,20 @@ class _MockCh934xSerialPlatform extends Ch934xSerialPlatform
   Future<bool> requestUsbPermission(int deviceId) async => true;
 
   @override
-  Future<Uint8List> read(int length, {int? serialPortIndex}) async => readBytes ?? Uint8List(0);
+  Future<Uint8List> read(int length, {int? serialPortIndex}) async =>
+      readBytes ?? Uint8List(0);
 
   @override
-  Future<int> write(Uint8List data) async => writeResult;
+  Future<int> write(Uint8List data, {int? serialPortIndex}) async {
+    lastWritePort = serialPortIndex;
+    return writeResult;
+  }
 
   @override
   Future<bool> setGpioOutput({
     required int gpioNumber,
     required int level,
-  }) async =>
-      gpioOutputResult;
+  }) async => gpioOutputResult;
 
   @override
   Future<int> getGpioInput(int gpioNumber) async => gpioInput;
@@ -105,13 +108,19 @@ class _MockCh934xSerialPlatform extends Ch934xSerialPlatform
   Future<void> setExceptionCallback(
     void Function(Ch934xException exception) onException,
   ) async {}
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   test('默认平台实现是 MethodChannelCh934xSerial', () {
-    expect(Ch934xSerialPlatform.instance, isInstanceOf<MethodChannelCh934xSerial>());
+    expect(
+      Ch934xSerialPlatform.instance,
+      isInstanceOf<MethodChannelCh934xSerial>(),
+    );
   });
 
   test('Ch934xSerial 委托 platform 调用 getDeviceList', () async {
@@ -131,6 +140,20 @@ void main() {
     expect(list.single.serialNumber, 'SN-1');
   });
 
+  test('Ch934xSerial write 透传可选串口索引', () async {
+    final mock = _MockCh934xSerialPlatform(writeResult: 3);
+    final plugin = Ch934xSerial.withPlatform(mock);
+
+    expect(await plugin.write(Uint8List.fromList([1, 2, 3])), 3);
+    expect(mock.lastWritePort, isNull);
+
+    expect(
+      await plugin.write(Uint8List.fromList([1, 2, 3]), serialPortIndex: 4),
+      3,
+    );
+    expect(mock.lastWritePort, 4);
+  });
+
   test('ModemStatus.isSet 按位与工作', () {
     const status = ModemStatus.cts | ModemStatus.dcd;
     expect(ModemStatus.isSet(status, ModemStatus.cts), isTrue);