Add unit test for ListViewItemStateImageIndexConverter file (#13716)

* Add unit test for ListViewItemStateImageIndexConverter file

* handle feedback

Author: Olina-Zhang

src/test/unit/System.Windows.Forms/System/Windows/Forms/ListViewItemStateImageIndexConverterTests.cs

@@ -1,8 +1,8 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
-#nullable disable
-
+using System.ComponentModel;
+using System.Drawing;
 using static System.ComponentModel.TypeConverter;
 
 namespace System.Windows.Forms.Tests;
@@ -25,4 +25,63 @@ public void ListViewItemStateImageIndexConverter_GetStandardValues_Null_Context_
 
         Assert.Empty(result);
     }
+
+    [Fact]
+    public void GetStandardValues_ContextWithoutListView_ReturnsEmpty()
+    {
+        object instance = new();
+        ITypeDescriptorContext context = new TypeDescriptorContextStub(instance);
+        ListViewItemStateImageIndexConverter converter = new();
+
+        StandardValuesCollection result = converter.GetStandardValues(context);
+
+        result.Cast<object>().Should().BeEmpty();
+    }
+
+    [Fact]
+    public void GetStandardValues_WithListViewWithoutStateImageList_ReturnsEmpty()
+    {
+        using ListView listView = new();
+        ListViewItem item = listView.Items.Add("test");
+
+        ITypeDescriptorContext context = new TypeDescriptorContextStub(item);
+        ListViewItemStateImageIndexConverter converter = new();
+
+        StandardValuesCollection result = converter.GetStandardValues(context);
+
+        result.Cast<object>().Should().BeEmpty();
+    }
+
+    [WinFormsFact]
+    public void GetStandardValues_WithStateImageList_ReturnsCorrectIndexes()
+    {
+        using ListView listView = new();
+        using ImageList imageList = new();
+        using Bitmap bmp1 = new(16, 16);
+        using Bitmap bmp2 = new(16, 16);
+        imageList.Images.Add(bmp1);
+        imageList.Images.Add(bmp2);
+        listView.StateImageList = imageList;
+
+        ListViewItem item = listView.Items.Add("test");
+
+        ITypeDescriptorContext context = new TypeDescriptorContextStub(item);
+        ListViewItemStateImageIndexConverter converter = new();
+
+        StandardValuesCollection result = converter.GetStandardValues(context);
+
+        result.Cast<object>().Should().Equal(0, 1);
+    }
+
+    // Minimal stub for ITypeDescriptorContext
+    private sealed class TypeDescriptorContextStub : ITypeDescriptorContext
+    {
+        public TypeDescriptorContextStub(object instance) => Instance = instance;
+        public object Instance { get; }
+        public IContainer? Container => null;
+        public object? GetService(Type serviceType) => null;
+        public void OnComponentChanged() { }
+        public bool OnComponentChanging() => false;
+        public PropertyDescriptor? PropertyDescriptor => null;
+    }
 }