code: protected function getBizContent()
      {
          $content = [
              'out_trade_no'    => $this->out_trade_no,
              'trade_no'        => $this->trade_no,
              'refund_amount'     => $this->refund_fee,
              'refund_reason'     => $this->reason,
              'out_request_no'    => $this->refund_no,
              'operator_id'       => $this->operator_id,
              'store_id'          => $this->store_id,
              'terminal_id'       => $this->terminal_id,
          ];
  
          return $content;
      }
docs: 业务请求参数的集合，最大长度不限，除公共参数外所有请求参数都必须放在这个参数中传递
  
  @return string