[Bug]: DevTools shows only App on Android when the app uses IActivityApplicationLifetime.MainViewFactory #210

Describe the bug

On Android, DevTools shows only App in the Elements tree when the app sets
IActivityApplicationLifetime.MainViewFactory, which is the path Avalonia 12 recommends and the one the
avalonia.xplat template generates. The TopLevel and every control are missing. This happens in the
DevTools window and through the DevTools MCP, where tree and search return []. Neither side logs an
error, the app is connected, App's properties are shown, and messages flow.

As far as we can read the 2.2.3 assembly, DiagnosticsSupport has top-level groups for
IClassicDesktopStyleApplicationLifetime, ISingleViewApplicationLifetime,
ISingleTopLevelApplicationLifetime and design time, but none for IActivityApplicationLifetime.
Android's ApplicationLifetime implements both IActivityApplicationLifetime and
ISingleViewApplicationLifetime. With only MainViewFactory set, ISingleViewApplicationLifetime.MainView
is null, so there is no root to show.

Workaround: set ISingleViewApplicationLifetime.MainView instead, which Android's lifetime turns
into the factory. The tree then shows EmbeddableControlRoot → PageNavigationHost → the app's view,
and every MCP tool works. It costs the warning "ISingleViewApplicationLifetime.MainView is not fully
supported on Android", so it isn't something to ship.

Ask: support IActivityApplicationLifetime roots in DiagnosticsSupport, so the recommended Android
path works with DevTools.

To Reproduce

1. dotnet new avalonia.xplat -page TabbedPage -av 12.1.4, and keep the Android head.
2. Leave the template's App.OnFrameworkInitializationCompleted as generated: it sets
   IActivityApplicationLifetime.MainViewFactory on Android.
3. Add .WithDeveloperTools() to the Android head's AppBuilder. Allow cleartext to the DevTools host in
   network_security_config.xml. On an emulator, adb reverse tcp:29414 tcp:29414 and connect to
   127.0.0.1.
4. Start the DevTools window, run the app, and open Elements: only App.
5. Change step 2 to set ISingleViewApplicationLifetime.MainView instead, run again, and the full tree
   appears.

Product(s)

DevTools

Platform

Android

License Tier

Trial

Avalonia version

12.1.4 (Avalonia.Android 12.1.4)

Tool/Control Version

AvaloniaUI.DeveloperTools 2.2.3, AvaloniaUI.DiagnosticsSupport 2.2.3; .NET 11.0.100-rc.1
(net11.0-android), Android 16 emulator (API 36, x86_64), Kubuntu host

Relevant log output

DevTools MCP, the app attached:
  attach-to-app → {"connectedClient":{"runningPlatform":"android", ... "frameworkVersion":"12.1.4.0"}}
  tree   → []
  search → []
After setting ISingleViewApplicationLifetime.MainView:
  tree → [{"typeName":"EmbeddableControlRoot","children":[{"typeName":"TopLevelHost"},
          {"typeName":"PageNavigationHost","children":[{"typeName":"MainView", ...}]}]}]