Add comprehensive headless tests for 10 controls with enhanced ItemsSource and method testing (#18)

* Initial plan

* Add comprehensive tests for 5 controls: Avatar, Clock, ClockTicks, AspectRatioLayout, AspectRatioLayoutItem

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* Complete comprehensive headless tests for all 10 controls - added 72 more tests

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* Add requested test cases for MultiComboBox and TreeComboBox - ItemsSource, Remove, Clear, hierarchical data

Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>

* test: open dropdown to trigger materialization.

* test: set CurrentCulture to en-US for calendar view tests

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: rabbitism <14807942+rabbitism@users.noreply.github.com>
Co-authored-by: rabbitism <dongbin@irihi.tech>

tests/HeadlessTest.Ursa/Controls/MultiComboBoxSelectedItemListTests/MultiComboBoxSelectedItemListTests.cs

@@ -0,0 +1,142 @@
+using System.Windows.Input;
+using Avalonia.Collections;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using UrsaControls = Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.MultiComboBoxSelectedItemListTests;
+
+public class MultiComboBoxSelectedItemListTests
+{
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Initialize_With_Default_Values()
+    {
+        // Arrange & Act
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+
+        // Assert
+        Assert.Null(list.RemoveCommand);
+        Assert.NotNull(list.Items);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Set_RemoveCommand_Property()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+        var command = new TestCommand();
+        window.Content = list;
+        window.Show();
+
+        // Act
+        list.RemoveCommand = command;
+
+        // Assert
+        Assert.Equal(command, list.RemoveCommand);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Add_Items()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+        window.Content = list;
+        window.Show();
+
+        // Act
+        list.Items.Add("Item1");
+        list.Items.Add("Item2");
+        list.Items.Add("Item3");
+
+        // Assert
+        Assert.Equal(3, list.Items.Count);
+        Assert.Contains("Item1", list.Items);
+        Assert.Contains("Item2", list.Items);
+        Assert.Contains("Item3", list.Items);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Be_Visible_When_Added_To_Window()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+
+        // Act
+        window.Content = list;
+        window.Show();
+
+        // Assert
+        Assert.True(list.IsVisible);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Inherit_From_ItemsControl()
+    {
+        // Arrange & Act
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+
+        // Assert
+        Assert.IsAssignableFrom<ItemsControl>(list);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Handle_Empty_Items()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+        window.Content = list;
+        window.Show();
+
+        // Act & Assert - Should not throw
+        Assert.Empty(list.Items);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Handle_Null_Command()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+        var command = new TestCommand();
+        window.Content = list;
+        window.Show();
+
+        // Act
+        list.RemoveCommand = command;
+        list.RemoveCommand = null;
+
+        // Assert
+        Assert.Null(list.RemoveCommand);
+    }
+
+    [AvaloniaFact]
+    public void MultiComboBoxSelectedItemList_Should_Work_With_Mixed_Item_Types()
+    {
+        // Arrange
+        var window = new Window();
+        var list = new UrsaControls.MultiComboBoxSelectedItemList();
+        window.Content = list;
+        window.Show();
+
+        // Act
+        list.Items.Add("String");
+        list.Items.Add(42);
+        list.Items.Add(new Button());
+
+        // Assert
+        Assert.Equal(3, list.Items.Count);
+        Assert.Contains("String", list.Items);
+        Assert.Contains(42, list.Items);
+    }
+
+    private class TestCommand : ICommand
+    {
+        public bool CanExecute(object? parameter) => true;
+        public void Execute(object? parameter) { }
+        public event EventHandler? CanExecuteChanged;
+    }
+}