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_method_channel_test.dart

@@ -69,13 +69,39 @@ void main() {
   test('write 将数据写入原生层并回传字节数', () async {
     mockCall((call) async {
       expect(call.method, 'write');
-      expect((call.arguments as Map)['data'], isA<Uint8List>());
+      expect(call.arguments, <String, Object>{
+        'data': Uint8List.fromList([9, 8, 7]),
+      });
       return 3;
     });
     final written = await platform.write(Uint8List.fromList([9, 8, 7]));
     expect(written, 3);
   });
 
+  test('write 透传显式串口索引', () async {
+    mockCall((call) async {
+      expect(call.method, 'write');
+      expect(call.arguments, <String, Object>{
+        'data': Uint8List.fromList([9, 8, 7]),
+        'serialPortIndex': 2,
+      });
+      return 3;
+    });
+    final written = await platform.write(
+      Uint8List.fromList([9, 8, 7]),
+      serialPortIndex: 2,
+    );
+    expect(written, 3);
+  });
+
+  test('write 接收负串口索引直接返回零', () async {
+    final written = await platform.write(
+      Uint8List.fromList([9]),
+      serialPortIndex: -1,
+    );
+    expect(written, 0);
+  });
+
   test('getModemStatus 默认回退到 0', () async {
     mockCall((_) async => null);
     expect(await platform.getModemStatus(), 0);