`AppWindow.Closing` event is not fired when calling `Application.Current.Exit()`

### Describe the bug

As the title: `AppWindow.Closing` event is not raised when calling `Application.Current.Exit()`

### Why is this important?

Since the event is not fired, it's impossible to track the window or cancel the window closing action with `AppWindowClosingEventArgs`

### Steps to reproduce the bug

1. Having this code
```cs
private void PART_WindowContent_Loaded(object sender, RoutedEventArgs e)
{
AppWindow.Closing += AppWindow_Closing;
AppWindow.Destroying += AppWindow_Destroying;
}

private void AppWindow_Closing(AppWindow sender, AppWindowClosingEventArgs e)
{
// this event is not fired
}

private void AppWindow_Destroying(AppWindow sender, object e)
{
// this event is fired
}
```
2. Call `Application.Current.Exit()` to quit the app while the window is open

### Actual behavior

`AppWindow.Closing` is not fired

### Expected behavior

`AppWindow.Closing` must be fired

### 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 provided AppWindow.Closing and Application.Current.Exit() reproduction on Windows App SDK 1.8.2. Trace the exit path and compare it with normal window closing, then verify that AppWindow.Closing is raised and remains cancellable before AppWindow.Destroying; no source file or test is named in the issue.

Written by the indexing model from the issue text.