[in_app_purchase_storekit] Fix consumable repurchase issue in StoreKit2

[ANONYMOUSZED-beep]

Fixes flutter/flutter#179111

Changes

• Fixed inish() method to always call the completion handler, even when the transaction is not found
• Added fallback to Transaction.unfinished when looking up transactions
• Added proper error handling with PigeonError for transaction finish failures
• Added tests for non-existent transaction finish and consumable repurchase scenarios

Problem

With StoreKit2, consumable products could only be purchased once because:

1. The inish() method would never call its completion handler if the transaction wasn't found in Transaction.all
2. This caused the Dart side to hang indefinitely
3. Consumable transactions needed to be found in Transaction.unfinished as well

Solution

• inish() now returns success even if the transaction is not found (it's effectively already complete)
• etchTransaction() now checks both Transaction.all and Transaction.unfinished
• Proper error handling added for unexpected failures

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[gemini-code-assist]

Code Review

This pull request effectively addresses a critical issue with repurchasing consumable products in StoreKit2. The changes to the finish and fetchTransaction methods are logical and well-implemented, ensuring that the completion handler is always called and that unfinished transactions are correctly handled. The addition of error handling and corresponding tests significantly improves the robustness of the implementation. I have a couple of suggestions to further enhance code maintainability and test reliability.

[gemini-code-assist]

Using a hardcoded transaction ID 0 makes this test fragile and dependent on the specific behavior of the test environment. A more robust approach would be to dynamically fetch the transaction created by the first purchase and use its actual ID. This ensures the test is more reliable and less likely to break with changes to the test session implementation.

    let transactionsExpectation = self.expectation(description: "Get transactions")
    var transactionId: Int64?
    plugin.transactions { result in
      switch result {
      case .success(let transactions):
        transactionId = transactions.first?.id
        transactionsExpectation.fulfill()
      case .failure(let error):
        XCTFail("Getting transactions should NOT fail. Failed with \(error)")
        transactionsExpectation.fulfill()
      }
    }
    await fulfillment(of: [transactionsExpectation], timeout: 5)

    guard let idToFinish = transactionId else {
      XCTFail("Could not get transaction ID to finish")
      return
    }

    // Finish the transaction
    plugin.finish(id: idToFinish) { result in
      switch result {
      case .success():
        finishExpectation.fulfill()
      case .failure(let error):
        XCTFail("Finish should NOT fail. Failed with \(error)")
      }
    }

    await fulfillment(of: [finishExpectation], timeout: 5)

[LongCatIsLooong]

The documentation says all should include unfinished consumables: https://developer.apple.com/documentation/storekit/transaction/all#Discussion. But if this method is only used by finish, shouldn't we use unfinished in first place instead of all?

[LongCatIsLooong]

This feels like a case that should be treated as an error since the programmer is trying to finish a non-existent transaction and we should not fail silently? Unless there're legitimate cases where developers can't tell if a transaction is finished or not.

[ANONYMOUSZED-beep]

Thanks for the review! I've addressed both points:

finish() now uses a new fetchUnfinishedTransaction() helper that only checks Transaction.unfinished
finish() now returns an error (storekit2_transaction_not_found) when the transaction is not found, instead of silently succeeding