修复 code review 问题：提取重复函数、补全缺失测试用例

- 提取 ResolveTermsFromTx 到 service/package，消除 order 和 auto_purchase 中的重复实现
- 补全 domain 测试：新增 from_activation 覆盖用例，现覆盖两种覆盖值
- 补全激活集成测试：新增 from_purchase 立即激活场景、历史记录兼容回退计数器验证
- 补全自动购包集成测试：新增购买后修改套餐配置不影响已有快照的验证

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/task/auto_purchase.go

@@ -703,21 +703,8 @@ func (h *AutoPurchaseHandler) activateAddonPackage(
 	return tx.Create(usage).Error
 }
 
-// resolvePackageTerms 在给定事务内查询分配覆盖并解析计时条款快照。
-// 必须在事务内调用，确保分配读取与 PackageUsage 写入在同一连接，避免快照与提交值不一致。
 func (h *AutoPurchaseHandler) resolvePackageTerms(ctx context.Context, tx *gorm.DB, pkg *model.Package, sellerShopID *uint) (packagedomain.TermsSnapshot, error) {
-	var allocation *model.ShopPackageAllocation
-	if sellerShopID != nil && *sellerShopID > 0 {
-		store := postgres.NewShopPackageAllocationStore(tx)
-		found, err := store.GetByShopAndPackageForSystem(ctx, *sellerShopID, pkg.ID)
-		if err != nil && err != gorm.ErrRecordNotFound {
-			return packagedomain.TermsSnapshot{}, pkgerrors.Wrap(pkgerrors.CodeDatabaseError, err, "查询套餐分配失败")
-		}
-		if err == nil {
-			allocation = found
-		}
-	}
-	return packagedomain.ResolveTermsSnapshot(pkg, allocation)
+	return packagepkg.ResolveTermsFromTx(ctx, tx, pkg, sellerShopID)
 }
 
 func parseLinkedPackageIDs(raw []byte) ([]uint, error) {