[in_app_purchase_storekit] Add Transaction.unfinished API and expose appAccountToken

[Haidar0096]

Summary

Adds two new StoreKit 2 features to in_app_purchase_storekit:

• SK2Transaction.unfinishedTransactions() - Queries only unfinished transactions for better performance
• SK2PurchaseDetails.appAccountToken - Exposes user UUID for backend integration

Motivation

1. Performance: Developers often only need unfinished transactions to complete them, not all historical transactions. This mirrors Apple's official Transaction.unfinished API.
2. User Identification: The ability to set appAccountToken already exists when making purchases, but reading it back from transaction details was missing.

Changes

• Added pigeon interface method for unfinishedTransactions()
• Implemented Swift native code using Apple's Transaction.unfinished API
• Exposed appAccountToken property in SK2PurchaseDetails
• Added unit tests for both features

Breaking Changes

None. Both features are additive and maintain full backward compatibility.

[gemini-code-assist]

Code Review

This pull request introduces two new features from StoreKit 2: the unfinishedTransactions() API and the exposure of appAccountToken on purchase details. The changes are additive, well-implemented, and follow the existing architecture. The implementation correctly spans native Swift code, pigeon interface updates, and Dart-side wrappers, complete with corresponding unit tests. This is a solid contribution that enhances the plugin's capabilities.

[gemini-code-assist]

The do block is unnecessary here because rawUnfinishedTransactions() is a non-throwing function. While it doesn't cause a bug in this case, it's a good practice to avoid do-catch structures when no errors can be thrown and caught. If rawUnfinishedTransactions were to throw, the lack of a catch block would prevent the completion handler from being called on an error path. Removing the do block simplifies the code and makes it more robust.

      @MainActor in
      let transactionsMsgs = await rawUnfinishedTransactions().map {
        $0.convertToPigeon(receipt: nil)
      }
      completion(.success(transactionsMsgs))

[Haidar0096]

This also partially solves #165355