[in_app_purchase_storekit] Use purchase(confirmIn:) for StoreKit 2 purchases - #13410

Fixes flutter/flutter#194373 Fixes flutter/flutter#195494

This PR switches StoreKit 2 purchases from the scene-less Product.purchase(options:) to Product.purchase(confirmIn:options:) whenever the plugin has a view controller attached to a window scene (iOS 17+) or an NSWindow (macOS 15.2+).

purchase(confirmIn:) is Apple's recommended purchase API for UIKit/AppKit apps. The
scene-less variant relies on StoreKit locating a UI anchor itself; on iOS 27 / iPadOS 27
that lookup fails (Could not find a UI anchor for … purchase) and the await never
returns. In the field this surfaced as App Review rejecting iPad subscription purchases
(#194373); on the iOS 27 simulator it surfaced as every StoreKit 2 purchase test timing
out (#195494). The scene-less call remains the fallback for iOS < 17 / macOS < 15.2 and
for headless engines whose registrar has no view in a window.

Test changes

The test setUp now hands the plugin a registrar stub backed by the test host's root
view controller, mirroring how the plugin runs inside a real Flutter app, so the
existing purchase tests exercise the confirmIn: path.

Two further adjustments were needed for the iOS 27 ipad simulator:

• testGetTransactionJsonRepresentation makes a purchase and then asks the plugin to look
  that transaction up by its ID. It took the ID from the test framework's
  SKTestTransaction.originalTransactionIdentifier, which only worked because, on earlier
  OS versions, that value happened to equal the StoreKit 2 Transaction.id the plugin
  matches on (both are 0 for the first purchase). On iOS 27 StoreKitTest numbers
  transactions differently (identifier is a non-zero counter, originalTransactionIdentifier
  is reported as 0), so the lookup found nothing. The test now uses identifier, which is the
  field that corresponds to Transaction.id on every OS version.
• testFailedProductUnavilablePurchase and testPurchaseWithIntroductoryOfferEligibilityJWS
  are skipped on iOS 27+. Both make a purchase that fails by design, and on iOS 27
  StoreKitTest shows an error dialog for failed purchases even with disableDialogs
  (iOS 27 release notes, known issue 184255116), so the purchase never completes. Since StoreKit serializes purchases per process, each of those also blocked every later purchase test, which is the cascade you see in the CI log.

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