Awaiting-validation stock check confirms orders for unknown product ids

The awaiting-validation handler looks up each ordered product with CatalogItems.Find and skips any id that comes back null; it rejects only when a found item lacks stock. So an order made entirely of unknown product ids gets OrderStockConfirmedIntegrationEvent. Nothing is reserved either: the paid handler later calls CatalogItem.RemoveStock, which throws CatalogDomainException once stock is zero.

Impact: An order for a nonexistent or already-removed product clears validation as confirmed, and the failure only surfaces later as an unhandled exception when the paid handler tries to remove stock, instead of as a rejection at the point the catalog actually knows the product id is unrecognised.

Recommendation (small effort): In the awaiting-validation handler, treat a null CatalogItems.Find result as a rejected line instead of skipping it, so an order of unrecognised products is rejected before payment.