docs: String getContentByCardId(String cardId) {
          Map<String, Object> request = new HashMap<>();
          request.put("card_id", cardId);
  
          String url = WxEndpoint.get("url.card.mpnews.gethtml");
          String json = JsonMapper.defaultMapper().toJson(request);
          logger.debug("get mpnews content by card id: {}", json);
          String response = wxClient.post(url, json);
          Map<String, Object> result = JsonMapper.defaultMapper().json2Map(response);
          if (result.containsKey("content")) {
              return result.get("content").toString();
          } else {
              throw new WxRuntimeException(999, "get html content by card id failed.");
          }
      }