feat: add headless tests.

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/FocusDialog.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             xmlns:u="https://irihi.tech/ursa"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus.FocusDialog">
+    <TextBox Text="Hello" u:FocusHelper.DialogFocusHint="True"></TextBox>
+</UserControl>

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/FocusDialog.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus;
+
+public partial class FocusDialog : UserControl
+{
+    public FocusDialog()
+    {
+        InitializeComponent();
+    }
+}

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/NormalDialog.axaml

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+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus.NormalDialog">
+    <TextBox Text="Hello"></TextBox>
+</UserControl>

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/NormalDialog.axaml.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+
+namespace HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus;
+
+public partial class NormalDialog : UserControl
+{
+    public NormalDialog()
+    {
+        InitializeComponent();
+    }
+}

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/Test.cs

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+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus;
+
+public class Test
+{
+    [AvaloniaFact]
+    public void Normal_Dialog_Focus_On_Border()
+    {
+        var ursaWindow = new TestWindow();
+        ursaWindow.Show();
+        ursaWindow.InvokeNormalDialog();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = ursaWindow.GetVisualDescendants().OfType<DefaultDialogControl>().FirstOrDefault();
+        Assert.NotNull(dialog);
+        var border = dialog.GetVisualDescendants().OfType<Border>().FirstOrDefault(a=>a.Name == "PART_Border");
+        var text = dialog.GetVisualDescendants().OfType<TextBox>().FirstOrDefault();
+        Assert.True(border?.IsFocused);
+        Assert.False(text?.IsFocused);
+    }
+    
+    [AvaloniaFact]
+    public void Focus_Dialog_Focus_On_Primary()
+    {
+        var ursaWindow = new TestWindow();
+        ursaWindow.Show();
+        ursaWindow.InvokeFocusDialog();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = ursaWindow.GetVisualDescendants().OfType<DefaultDialogControl>().FirstOrDefault();
+        Assert.NotNull(dialog);
+        var border = dialog.GetVisualDescendants().OfType<Border>().FirstOrDefault(a=>a.Name == "PART_Border");
+        var text = dialog.GetVisualDescendants().OfType<TextBox>().FirstOrDefault();
+        Assert.False(border?.IsFocused);
+        Assert.True(text?.IsFocused);
+    }
+}

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/TestWindow.axaml

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+<u:UrsaWindow xmlns="https://github.com/avaloniaui"
+            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+            xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+            xmlns:u="https://irihi.tech/ursa"
+            mc:Ignorable="d" d:DesignWidth="800"
+            d:DesignHeight="450"
+            x:Class="HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus.TestWindow"
+            Title="TestWindow">
+    Welcome to Avalonia!
+</u:UrsaWindow>

tests/HeadlessTest.Ursa/Controls/OverlayShared/Dialog_Primary_Focus/TestWindow.axaml.cs

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+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.OverlayShared.Dialog_Primary_Focus;
+
+public partial class TestWindow : UrsaWindow
+{
+    public TestWindow()
+    {
+        InitializeComponent();
+    }
+
+    public void InvokeNormalDialog()
+    {
+        OverlayDialog.ShowModal<NormalDialog, object>("Hello World",
+            options: new OverlayDialogOptions { TopLevelHashCode = GetHashCode() });
+    }
+
+    public void InvokeFocusDialog()
+    {
+        OverlayDialog.ShowModal<FocusDialog, object>("Hello World",
+            options: new OverlayDialogOptions { TopLevelHashCode = GetHashCode() });
+    }
+}