[Shell] Shell currently doesn't detect pushed pages inside a Modal NavigationPage

```C#
[Test]
public async Task DotDotNavigationWorksWithModalNavigationPage()
{
var shell = new TestShell();
shell.Items.Add(new ContentPage());
var modalPage = new Controls.NavigationPage(new ContentPage());

await shell.Navigation.PushModalAsync(modalPage);
await modalPage.PushAsync(new ContentPage());
await modalPage.PushAsync(new ContentPage());
await modalPage.PushAsync(new ContentPage());
await shell.GoToAsync("..");
}
```

Start with the DotDotNavigationWorksWithModalNavigationPage test and trace shell.Navigation.PushModalAsync, modalPage.PushAsync, and shell.GoToAsync(".."). Determine the expected back-navigation result for the pushed pages, then add regression coverage showing that modal NavigationPage state is detected correctly.

Written by the indexing model from the issue text.