Browser does not redirect back to app after logout (postLogoutRedirectUrl does not work)

When doing:
```
await _appAuth.endSession(EndSessionRequest(
idTokenHint: idToken,
postLogoutRedirectUrl: postLogoutRedirectUrl,
serviceConfiguration: serviceConfiguration,
externalUserAgent: externalUserAgent
));
```

The app triggers the browser and successfully logs out the user. However, after that, it does not automatically redirect the user back to the app, as it does with login.

I can see that in the [example project](https://github.com/MaikuB/flutter_appauth/tree/master/flutter_appauth/example) the logout redirects the user to a website where there is a manual button to return to the app. In my case, I want to redirect the user directly to the app without having a button on the logout browser page.

Because of this, the only option is to have the user manually exit the browser to return to the app, which triggers `AppAuth Error: The user cancelled the flow!` and does not get information back on whether the logout was successful or not.

Start with the example project and the EndSessionRequest usage shown in the issue, then trace how the Android and iOS flows handle postLogoutRedirectUrl. Compare the logout callback behavior with login and determine whether the app receives a direct return and logout result without manual browser exit.

Written by the indexing model from the issue text.