test: Add sample for nested ItemsView

src/SamplesApp/UITests.Shared/Microsoft_UI_Xaml_Controls/ItemsViewTests/ItemsView_Nested.xaml

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+<Page x:Class="UITests.Microsoft_UI_Xaml_Controls.ItemsViewTests.ItemsView_Nested"
+	  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+	  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+	  xmlns:local="using:UITests.Microsoft_UI_Xaml_Controls.ItemsViewTests"
+	  xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+	  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+	  xmlns:winui="using:Microsoft.UI.Xaml.Controls"
+	  mc:Ignorable="d"
+	  Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
+
+	<StackPanel>
+		<ItemsRepeater ItemsSource="{x:Bind Categories, Mode=OneWay}">
+			<ItemsRepeater.ItemTemplate>
+				<DataTemplate x:DataType="local:Category">
+					<ItemContainer>
+						<StackPanel DataContext="{x:Bind}">
+
+							<TextBlock Text="{x:Bind Label}"
+									   HorizontalAlignment="Center"
+									   FontSize="20"
+									   Margin="0,10,0,10" />
+
+							<ItemsView ItemsSource="{x:Bind}">
+								<ItemsView.ItemTemplate>
+									<DataTemplate x:DataType="local:Item">
+										<ItemContainer>
+											<TextBlock Text="{x:Bind Label}"
+													   HorizontalAlignment="Center" />
+										</ItemContainer>
+									</DataTemplate>
+								</ItemsView.ItemTemplate>
+							</ItemsView>
+
+						</StackPanel>
+					</ItemContainer>
+				</DataTemplate>
+			</ItemsRepeater.ItemTemplate>
+		</ItemsRepeater>
+
+		<ItemsView ItemsSource="{x:Bind Categories}">
+			<ItemsView.ItemTemplate>
+				<DataTemplate x:DataType="local:Category">
+					<ItemContainer>
+						<TextBlock Text="{x:Bind Label}"
+								   HorizontalAlignment="Center" />
+					</ItemContainer>
+				</DataTemplate>
+			</ItemsView.ItemTemplate>
+		</ItemsView>
+	</StackPanel>
+</Page>

src/SamplesApp/UITests.Shared/Microsoft_UI_Xaml_Controls/ItemsViewTests/ItemsView_Nested.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices.WindowsRuntime;
+using Uno.UI.Samples.Controls;
+using Windows.Foundation;
+using Windows.Foundation.Collections;
+using Microsoft.UI.Xaml;
+using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Controls.Primitives;
+using Microsoft.UI.Xaml.Data;
+using Microsoft.UI.Xaml.Input;
+using Microsoft.UI.Xaml.Media;
+using Microsoft.UI.Xaml.Navigation;
+using System.ComponentModel;
+using System.Collections.ObjectModel;
+
+namespace UITests.Microsoft_UI_Xaml_Controls.ItemsViewTests
+{
+	[Sample("ItemsView", IgnoreInSnapshotTests = true)]
+	public sealed partial class ItemsView_Nested : Page
+	{
+		List<Category> Categories { get; set; }
+
+		public ItemsView_Nested()
+		{
+			InitializeComponent();
+
+			Categories = GenerateRandomCategories(5, 10);
+
+			Categories[0].Collapsed = !Categories[0].Collapsed;
+		}
+
+		private static List<Category> GenerateRandomCategories(int categoryCount, int maxItemsPerCategory)
+		{
+			var random = new Random();
+			var categories = new List<Category>();
+
+			for (int i = 1; i <= categoryCount; i++)
+			{
+				var itemCount = random.Next(1, maxItemsPerCategory + 1);
+				var items = Enumerable.Range(1, itemCount)
+									  .Select(j => new Item($"Item {j}"))
+									  .ToList();
+				var category = new Category($"Category {i}", items, random.Next(0, 2) == 0);
+				categories.Add(category);
+			}
+
+			return categories;
+		}
+	}
+
+	public partial class Item(string label)
+	{
+		public string Label { get; } = label;
+	}
+
+	public partial class Category : ObservableCollection<Item>
+	{
+		private bool _collapsed;
+
+		private readonly List<Item> _collapsedItems = [];
+
+		public string Label { get; }
+
+		public bool Collapsed
+		{
+			get => _collapsed;
+			set
+			{
+				if (_collapsed != value)
+				{
+					if (_collapsed)
+					{
+						foreach (Item item in _collapsedItems)
+							Add(item);
+						_collapsedItems.Clear();
+					}
+					else
+					{
+						_collapsedItems.AddRange(Items);
+						Clear();
+					}
+					_collapsed = value;
+				}
+			}
+		}
+
+		public Category(string label, IEnumerable<Item> items, bool collapsed = false) : base(items)
+		{
+			Label = label;
+			Collapsed = collapsed;
+		}
+	}
+}