ContentDialog focus highlight is shown when dialog is opened via a keyboard event

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### Describe the bug

When a ContentDialog is shown which contains interactive controls, the display of the focus highlight is inconsistent:

1. If the dialog is opened via a button click, the highlight is not shown. The first interactive control on the dialog is focussed, but the highlight is not shown.
2. If the dialog is opened via a `KeyUp` event handler or in response to a `KeyboardAccelerator` invocation, the focus highlight _is_ shown.

### Steps to reproduce the bug

1. Create a new project in Visual Studio. Choose "Blank App, Packaged (WinUI 3 in Desktop)" as the project template.

2. Name the project "CheckboxTest".

3. Replace the `MainWindow.xaml` file contents with the XAML below:

``` xaml

```

4. Replace the `MainWindow.xaml.cs` file code with:

``` csharp
using System;
using System.Threading.Tasks;
using Microsoft.UI.Xaml;
using Microsoft.UI.Xaml.Controls;
using Microsoft.UI.Xaml.Input;

namespace CheckboxTest;

public sealed partial class MainWindow : Window
{
public MainWindow()
{
this.InitializeComponent();
this.Content.KeyUp += Content_KeyUp;
}

private async void Content_KeyUp(object sender, KeyRoutedEventArgs e)
{
if (e.Key == Windows.System.VirtualKey.Delete)
{
if (await OpenDeleteDialog() == ContentDialogResult.Primary)
{
// ... do delete stuff
}
}
}

private async void ShowDialogButton_Click(object sender, RoutedEventArgs e)
{
if (await OpenDeleteDialog() == ContentDialogResult.Primary)
{
// ... do delete stuff
}
}

private async Task OpenDeleteDialog()
{
DeleteConfirmationDialog.XamlRoot = Content.XamlRoot;
return await DeleteConfirmationDialog.ShowAsync();
}
}
```

5. Run the project and click the "Show Dialog" button. Observe that the dialog is shown with no focus highlight present on the checkbox control:


6. Close the dialog by pressing Enter or selecting the "Cancel" button.

7. Press Delete. Observe that the dialog is shown with the first interactive element highlighted as the focussed control:


### Expected behavior

The display of the focus highlight should be consistent, i.e. independent of the method used to show the ContentDialog. I suggest not showing the highlight until the user uses Tab to move the focus around the dialog's controls.

### Screenshots

_No response_

### NuGet package version

None

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

Additional context may be found on my StackOverflow question here: https://stackoverflow.com/questions/79376328/winui-3-how-to-set-focus-to-a-contentdialog-button/79382988

Reproduce the behavior using the MainWindow.xaml and MainWindow.xaml.cs samples, starting at OpenDeleteDialog and ContentDialog.ShowAsync. Compare opening the dialog from ShowDialogButton_Click with Content_KeyUp and the Delete keyboard event. Done means the focus highlight behavior is consistent for both opening paths and matches the stated keyboard-navigation expectation.

Written by the indexing model from the issue text.