Add missing POSItemIdentifier import to PointOfSaleItemsControllerTests

Fixes compilation errors where POSItemIdentifier was not found in scope.

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Co-Authored-By: Claude <[email protected]>

Author: joshheald

Modules/Tests/PointOfSaleTests/Controllers/PointOfSaleItemsControllerTests.swift

@@ -3,6 +3,7 @@ import Foundation
 @testable import PointOfSale
 import struct Yosemite.POSVariableParentProduct
 import enum Yosemite.POSItem
+import struct Yosemite.POSItemIdentifier
 import enum Yosemite.PointOfSaleItemServiceError
 import class Yosemite.PointOfSaleItemFetchStrategyFactory
 @testable import struct Yosemite.PointOfSaleSearchPurchasableItemFetchStrategy