Handles keep increasing when repeatedly showing/closing a Window on a separate Dispatcher thread

### Description

In a WPF application, when starting a dedicated STA thread running Dispatcher.Run() and repeatedly creating, showing, and closing a Window on that thread, the process handle count keeps increasing.
The handle type is File, and the FileName points to the WPF runtime DLLs, for example:
```
Type: File
FileName: C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App\8.0.22
```

Closing the Window does not release these handles. They are only released when the Dispatcher is shut down.

### Reproduction Steps

1. Create a .NET 8 WPF application.
2. Start a dedicated UI thread and run a Dispatcher as shown below:

```cs
private Dispatcher? _dispatcher;
private Thread? _thread;

private Dispatcher EnsureDispatcher()
{
if (_dispatcher == null)
{
using var evt = new AutoResetEvent(false);
_thread = new Thread(_ =>
{
_dispatcher = Dispatcher.CurrentDispatcher;
evt.Set();
Dispatcher.Run();
});
_thread.IsBackground = true;
_thread.SetApartmentState(ApartmentState.STA);
_thread.Start();

evt.WaitOne();
}

return _dispatcher!;
}
```

3. On this Dispatcher, repeatedly create a Window, call Show(), then Close().
4. Monitor the process using Task Manager or Process Explorer. The number of File handles increases continuously.

### Expected behavior

After calling Window.Close(), unnecessary handles should be released. Handle count should not grow indefinitely.

### Actual behavior

Handles are not released after closing the Window. They accumulate with each show/close cycle.
Calling Dispatcher.InvokeShutdown() releases them.

```cs
public void Dispose()
{
_dispatcher?.BeginInvokeShutdown(DispatcherPriority.Normal);

if (_thread != null && _thread.IsAlive)
{
_thread.Join();
}
_dispatcher = null;
_thread = null;
_runtime = null;
}
```

### Regression?

_No response_

### Known Workarounds

_No response_

### Impact

_No response_

### Configuration

- .NET: 8.0.22
- OS: Windows 11 22H2
- WPF: Microsoft.WindowsDesktop.App 8.0.22

### Other information

- The same behavior was observed on .NET 10 preview as well.