fix: 补上必填字段 discount

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

service/src/main/java/com/kym/service/wechat/impl/WechatPayFapiaoService.java

@@ -145,6 +145,7 @@ public class WechatPayFapiaoService {
         item.put("total_amount", totalAmount);
         item.put("tax_rate", taxRate);
         item.put("tax_prefer_mark", "NO_FAVORABLE");
+        item.put("discount", false);
         return item;
     }
 }