fix(pricing): 修复优惠券计算逻辑，防止优惠金额溢出

- 重构优惠券折扣计算方法，确保固定金额优惠券不超过适用商品总价
- 修改百分比优惠券计算逻辑，基于适用商品总价而非购物车总价
- 新增适用商品总价计算方法，支持按商品类型过滤
- 添加防止优惠金额超出适用商品总价的保护逻辑
- 完善无商品类型限制时的全商品适用逻辑
- 增加多种边界情况和多SKU场景的单元测试
- 修复百分比优惠券最大折扣限制的计算顺序问题

src/main/java/com/ycwl/basic/pricing/service/impl/CouponServiceImpl.java

@@ -70,17 +70,27 @@ public class CouponServiceImpl implements ICouponService {
         if (!isCouponApplicable(coupon, products, totalAmount)) {
             return BigDecimal.ZERO;
         }
-        
+
+        // 计算适用商品的总价
+        BigDecimal applicableProductsTotal = calculateApplicableProductsTotal(coupon, products);
+
         BigDecimal discount;
         if (coupon.getCouponType() == CouponType.PERCENTAGE) {
-            discount = totalAmount.multiply(coupon.getDiscountValue().divide(BigDecimal.valueOf(100), 4, RoundingMode.HALF_UP));
+            // 百分比优惠券基于适用商品总价计算,而非购物车总价
+            discount = applicableProductsTotal.multiply(coupon.getDiscountValue().divide(BigDecimal.valueOf(100), 4, RoundingMode.HALF_UP));
             if (coupon.getMaxDiscount() != null && discount.compareTo(coupon.getMaxDiscount()) > 0) {
                 discount = coupon.getMaxDiscount();
             }
         } else {
+            // 固定金额优惠券
             discount = coupon.getDiscountValue();
         }
-        
+
+        // 限制优惠金额不超过适用商品总价,防止优惠溢出到其他SKU
+        if (discount.compareTo(applicableProductsTotal) > 0) {
+            discount = applicableProductsTotal;
+        }
+
         return discount.setScale(2, RoundingMode.HALF_UP);
     }
     
@@ -128,7 +138,37 @@ public class CouponServiceImpl implements ICouponService {
             return false;
         }
     }
-    
+
+    /**
+     * 计算适用商品的总价
+     * @param coupon 优惠券配置
+     * @param products 商品列表
+     * @return 适用商品的总价
+     */
+    private BigDecimal calculateApplicableProductsTotal(PriceCouponConfig coupon, List<ProductItem> products) {
+        // 如果优惠券没有商品类型限制,返回所有商品总价
+        if (coupon.getApplicableProducts() == null || coupon.getApplicableProducts().isEmpty()) {
+            return products.stream()
+                    .map(ProductItem::getSubtotal)
+                    .reduce(BigDecimal.ZERO, BigDecimal::add);
+        }
+
+        try {
+            // 解析适用商品类型列表
+            List<String> applicableProductTypes = objectMapper.readValue(
+                    coupon.getApplicableProducts(), new TypeReference<List<String>>() {});
+
+            // 计算适用商品的总价
+            return products.stream()
+                    .filter(product -> applicableProductTypes.contains(product.getProductType().getCode()))
+                    .map(ProductItem::getSubtotal)
+                    .reduce(BigDecimal.ZERO, BigDecimal::add);
+        } catch (Exception e) {
+            log.error("解析适用商品类型失败", e);
+            return BigDecimal.ZERO;
+        }
+    }
+
     @Override
     @Transactional
     public CouponUseResult useCoupon(CouponUseRequest request) {