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_terms_integration_test.go

@@ -136,6 +136,30 @@ func TestAutoPurchaseTransactionRollbackOnInvalidAddonTerms(t *testing.T) {
 	}
 }
 
+// TestAutoPurchaseConfigChangeDoesNotModifySnapshot 验证自动购包后修改套餐配置不改变已有使用记录的计时快照。
+func TestAutoPurchaseConfigChangeDoesNotModifySnapshot(t *testing.T) {
+	tx := testutil.NewPostgresTransaction(t)
+	card := createAutoPurchaseCard(t, tx, constants.RealNameStatusVerified)
+	pkg := createAutoPurchasePackage(t, tx, constants.PackageTypeFormal, constants.PackageExpiryBaseFromPurchase, 30)
+	handler := &AutoPurchaseHandler{db: tx, shopPackageAllocationStore: postgres.NewShopPackageAllocationStore(tx), logger: zap.NewNop()}
+	order := newAutoPurchaseOrder(card.ID, nil)
+	if err := handler.activateMainPackage(context.Background(), tx, order, pkg, constants.AssetWalletResourceTypeIotCard, card.ID, time.Now()); err != nil {
+		t.Fatalf("自动购包创建主套餐失败：%v", err)
+	}
+	// 购买后修改套餐的生效条件和时长
+	if err := tx.Model(pkg).Updates(map[string]any{"expiry_base": constants.PackageExpiryBaseFromActivation, "duration_days": 99}).Error; err != nil {
+		t.Fatalf("修改套餐配置失败：%v", err)
+	}
+	var usage model.PackageUsage
+	if err := tx.Where("order_id = ? AND package_id = ?", order.ID, pkg.ID).First(&usage).Error; err != nil {
+		t.Fatalf("查询自动购包记录失败：%v", err)
+	}
+	// 快照应保留购买时的值，不随套餐修改而变化
+	if usage.ExpiryBaseSnapshot != constants.PackageExpiryBaseFromPurchase || usage.DurationDaysSnapshot != 30 {
+		t.Fatalf("套餐配置变更不应影响已购买快照：expiryBase=%s days=%d", usage.ExpiryBaseSnapshot, usage.DurationDaysSnapshot)
+	}
+}
+
 func createAutoPurchaseCard(t *testing.T, tx *gorm.DB, realnameStatus int) *model.IotCard {
 	t.Helper()
 	suffix := strconv.FormatInt(time.Now().UnixNano()%100000000000000000, 10)