[in_app_purchase_storekit] Expose quantity in Transactions

[LouiseHsu]

Fixes flutter/flutter#186490

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

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[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.