docs: public String addMpNewsImage(InputStream inputStream, String fileName) {
          String url = WxEndpoint.get("url.mpnews.image.upload");
          String response = wxClient.post(url, inputStream, fileName);
  
          Map<String, Object> result = JsonMapper.defaultMapper().json2Map(response);
          if (result.containsKey("url")) {
              return result.get("url").toString();
          } else {
              logger.warn("mpnews image upload failed: {}", response);
              throw new WxRuntimeException(999, response);
          }
      }