getAppAccountToken method
Return the current identity scope's Apple appAccountToken (a
UUID), creating and persisting one on first call. Pass this to
StoreKit's purchase options so the resulting transaction carries the
token Apple echoes back — the anchor the receipt/server-to-server
notification path uses to bind the transaction to this subscriber.
logOut clears it, so a new anonymous scope gets its own token.
Implementation
Future<String> getAppAccountToken() => _platform.getAppAccountToken();