IsTabStop is true by default on TitleBar

Nobody has claimed this yet.

### Describe the bug

`IsTabStop` is `true` by default on TitleBar which is strange in my option. There is nothing you can to when focus is set on the TitleBar, nor you can see it's focused.

### Why is this important?

It's makes tab navigation confusing.

### Steps to reproduce the bug

1. Create a new blank WinUI application.
2. In MainWindow.xaml, set this as the content of the Window:

```xaml

```

3. Set this as the constructor:

```C#
public MainWindow()
{
InitializeComponent();

ExtendsContentIntoTitleBar = true;
SetTitleBar(AppTitleBar);
}
```

### Actual behavior

When you press tab you should navigate just between the `Button` and the `TextBox`.

### Expected behavior

When you press tab you should navigate just between the `Button` and the `TextBox` and the `TitleBar`.

### Screenshots

Button in focus:

TextBox in focus:

TitleBar in focus:

### NuGet package version

Microsoft.WindowsAppSDK 2.3.1

### Windows version

Windows 11 (26H1): Build 28000

### Additional context

Issue is easily solved by adding `IsTabStop="False"` to `TitleBar`.

Start with the MainWindow.xaml reproduction and the MainWindow constructor, focusing on the TitleBar's IsTabStop behavior. Inspect the TitleBar control entry point and any existing related tests; done means tab navigation no longer stops on the TitleBar by default while Button and TextBox navigation remains intact.

Written by the indexing model from the issue text.