AppWindow does not support IWeakReferenceSource

### Describe the bug

This is mainly for C++WinRT.

### Why is this important?

This will make getting a weak_ref of AppWindow crash
```cpp
auto ref = winrt::make_weak(AppWindow());
```

As a result, using `winrt::auto_revoke` on any `AppWindow` events will crash tpp
```cpp
auto revoker = appWindow.Changed(winrt::auto_revoke, [](auto&&... args){ });
```

Meanwhile this usage is [clearly documented](https://learn.microsoft.com/en-us/windows/windows-app-sdk/api/winrt/microsoft.ui.windowing.appwindow.changed?view=windows-app-sdk-1.8#microsoft-ui-windowing-appwindow-changed)

### Steps to reproduce the bug

As above

### Actual behavior

Crash

### Expected behavior

Works

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.8.2: 1.8.251003001

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Start with the AppWindow implementation and reproduce the failure using winrt::make_weak(AppWindow()) and the winrt::auto_revoke event example from the issue. Done means both weak-reference creation and AppWindow event revocation work without crashing for C++/WinRT users.

Written by the indexing model from the issue text.