fix: 微信回复消息 Content-Type 添加 UTF-8 字符集，修复中文乱码

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

car-wash-service/src/main/java/com/kym/service/wechat/impl/WeixinMPServiceImpl.java

@@ -139,7 +139,7 @@ public class WeixinMPServiceImpl implements WeixinMPService {
                     }
                 }
 
-                response.setContentType("text/html");
+                response.setContentType("text/html; charset=UTF-8");
                 result = XmlUtil.mapToXmlStr(ret);
             }
         } catch (Exception e) {