Back-in-stock notifications not sent for attribute combinations with multiple warehouses #881

Description

When a product uses stock management by attribute combinations with multiple warehouses, raising the stock of a combination's warehouse in the admin panel does not send back-in-stock notifications to subscribers.

Steps to reproduce

1. Create a product, published, with Manage inventory method = Manage stock by attributes, Use multiple warehouses enabled, Allow out of stock subscriptions enabled and Backorders = No backorders.
2. Add an attribute combination with warehouse stock 0 in one warehouse.
3. As a customer, subscribe to the "notify me when available" for that combination.
4. In admin, edit the combination (edit popup) and raise that warehouse's stock above 0.
5. No notification is queued for the subscriber.

Cause

ProductViewModelService.InsertOrUpdateProductAttributeCombinationPopup (src/Web/Grand.Web.AdminShared/Services/ProductViewModelService.cs, ~line 2239) snapshots the previous state with combination.Clone(). ProductAttributeCombination.Clone() (src/Core/Grand.Domain/Catalog/ProductAttributeCombination.cs, line 87) is MemberwiseClone(), a shallow copy, so the WarehouseInventory list and its items are shared between the clone and the original.

PrepareCombinationWarehouseInventory (~line 2088) then modifies those shared ProductCombinationWarehouseInventory items in place (existingPwI.StockQuantity = ..., and Add/Remove on the shared list). When OutOfStockNotifications(product, combination, prevCombination) runs (~line 223), the multi-warehouse branch compares prevcombination.WarehouseInventory with combination.WarehouseInventory, which are the same objects, so "previously out of stock, now in stock" is never detected, both for the per-warehouse check and for the total-sum check.

The single-warehouse branch is not affected, since StockQuantity is a value type copied by the clone. The product-level path is not affected either: it snapshots prevMultiWarehouseStock into new ProductWarehouseInventory objects (~line 781).

Suggested fix

Deep-copy WarehouseInventory when snapshotting the previous state (e.g. in Clone() or in the service, create new ProductCombinationWarehouseInventory items with WarehouseId, StockQuantity, ReservedQuantity), or capture the previous per-warehouse quantities before PrepareCombinationWarehouseInventory mutates them. A unit test for the combination edit path with multiple warehouses would cover this.

Notes

Related: #880 (journaling manual stock changes), which touches the same manual stock edit flow.