fix: menuGet() 返回 WxMpMenu，用 fastjson 序列化为 String

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

car-wash-miniapp/src/main/java/com/kym/miniapp/controller/WeixinMPController.java

@@ -9,6 +9,7 @@ import com.kym.common.R;
 import com.kym.service.MpRelationService;
 import com.kym.service.wechat.WeixinMPService;
 import jakarta.servlet.http.HttpServletRequest;
+import com.alibaba.fastjson2.JSON;
 import me.chanjar.weixin.mp.api.WxMpService;
 import jakarta.servlet.http.HttpServletResponse;
 import lombok.extern.slf4j.Slf4j;
@@ -85,8 +86,8 @@ public class WeixinMPController {
     @ResponseBody
     public R<?> exportMenu() {
         try {
-            String menuJson = wxMpService.getMenuService().menuGet();
-            return R.success(menuJson);
+            var menu = wxMpService.getMenuService().menuGet();
+            return R.success(JSON.toJSONString(menu));
         } catch (WxErrorException e) {
             log.error("导出菜单失败", e);
             return R.failed(-1, "导出失败: " + e.getMessage());