[in_app_purchase_storekit] Expose quantity in Transactions

[LouiseHsu]

Fixesflutter/flutter#186490

Part of an attempt to one-shot iap issues, inspired by camera work for flutter android

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[gemini-code-assist]

Code Review

This pull request updates the in_app_purchase_storekit package to expose the actual purchased quantity in SK2Transaction by mapping purchasedQuantity from SK2TransactionMessage. It also updates the fake platform and adds tests to verify this behavior. Review feedback suggests using a null-coalescing operator when assigning purchasedQuantity to prevent null-safety compilation errors, and initializing list variables directly in the fake platform instead of using late to avoid potential LateInitializationErrors.