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/service/package/usage_terms.go

@@ -1,12 +1,15 @@
 package packagepkg
 
 import (
+	"context"
 	"sync/atomic"
 
 	packagedomain "github.com/break/junhong_cmp_fiber/internal/domain/package"
 	"github.com/break/junhong_cmp_fiber/internal/model"
+	"github.com/break/junhong_cmp_fiber/internal/store/postgres"
 	"github.com/break/junhong_cmp_fiber/pkg/errors"
 	"go.uber.org/zap"
+	"gorm.io/gorm"
 )
 
 var historicalTermsFallbackCount atomic.Uint64
@@ -41,6 +44,23 @@ func HistoricalTermsFallbackCount() uint64 {
 	return historicalTermsFallbackCount.Load()
 }
 
+// ResolveTermsFromTx 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 ResolveTermsFromTx(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{}, errors.Wrap(errors.CodeDatabaseError, err, "查询套餐分配失败")
+		}
+		if err == nil {
+			allocation = found
+		}
+	}
+	return packagedomain.ResolveTermsSnapshot(pkg, allocation)
+}
+
 func isEmptyHistoricalTerms(usage *model.PackageUsage) bool {
 	return usage.ExpiryBaseSnapshot == "" && usage.CalendarTypeSnapshot == "" &&
 		usage.DurationMonthsSnapshot == 0 && usage.DurationDaysSnapshot == 0