No way to keep the browser open for nested SSO with authenticator app

Nobody has claimed this yet.

Flow:
1. The Flutter app opens the IDP authenticate url in a Chrome Custom Tab.
2. The user presses the one tap login button which opens the authenticator app (using a custom scheme) where the user approves the sign in request
3. The authenticator app calls the Flutter app's callback scheme
4. The IDP page inside the Chrome Custom Tab activity checks periodically whether the login is approved to execute a callback with the 'code' parameter

Request: The in app browser activity should remain open within (on top of?) the Flutter app not only until the app receives **any** callback, but until a specific parameter (code) is present in the callback.

Start by tracing the Flutter plugin's handling of the Chrome Custom Tab lifecycle and callback scheme described in the issue. Check how it currently decides that authentication is complete, then verify that the browser remains open through the authenticator-app round trip and closes only when the callback contains the `code` parameter.

Written by the indexing model from the issue text.