fix: rejectRefund 不再清除 refundAmount，避免累计已退款金额丢失

refundAmount 记录的是累计已成功退款的金额，拒绝本次申请时
不应清空，否则之前的成功部分退款记录会丢失。

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

haha-service/src/main/java/com/haha/service/impl/RefundServiceImpl.java

@@ -308,12 +308,11 @@ public class RefundServiceImpl extends ServiceImpl<RefundMapper, Refund> impleme
         refund.setUpdateTime(LocalDateTime.now());
         this.updateById(refund);
 
-        // 清除订单退款状态
+        // 清除订单退款状态（refundAmount 保留，因为它记录的是累计已成功退款金额）
         Order order = orderService.getById(refund.getOrderId());
         if (order != null) {
             order.setRefundStatus(null);
             order.setCurrentRefundId(null);
-            order.setRefundAmount(null);
             order.setRefundReason(null);
             orderService.updateById(order);
         }