Add unit tests for LinkAreaEditor.LinkAreaUI (#12725)

Related #10773

Proposed changes
Add unit test LinkAreaEditor.LinkAreaUITests.cs for public properties and method of the LinkAreaEditor.LinkAreaUI.
Enable nullability in LinkAreaEditor.LinkAreaUITests.cs.

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/LinkAreaEditor.LinkAreaUITests.cs

@@ -0,0 +1,116 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using Moq;
+using System.ComponentModel.Design;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class LinkAreaEditorLinkAreaUITests : IDisposable
+{
+    private readonly LinkAreaEditor.LinkAreaUI _linkAreaUI;
+    private readonly Mock<IHelpService> _mockHelpService;
+
+    public LinkAreaEditorLinkAreaUITests()
+    {
+        _mockHelpService = new();
+        _linkAreaUI = new(_mockHelpService.Object);
+    }
+
+    public void Dispose()
+    {
+        _linkAreaUI.Dispose();
+    }
+
+    [Fact]
+    public void LinkAreaUI_Constructor_InitializesCorrectly()
+    {
+        _linkAreaUI.Should().NotBeNull();
+        _linkAreaUI.Should().BeOfType<LinkAreaEditor.LinkAreaUI>();
+
+        IHelpService? helpServiceField = _linkAreaUI.TestAccessor().Dynamic._helpService;
+
+        helpServiceField.Should().Be(_mockHelpService.Object);
+    }
+
+    [Fact]
+    public void SampleText_SetGet_WorksCorrectly()
+    {
+        string sampleText = "Test Sample Text";
+        _linkAreaUI.SampleText = sampleText;
+
+        _linkAreaUI.SampleText.Should().Be(sampleText);
+    }
+
+    [Fact]
+    public void SampleText_Set_UpdatesSelection()
+    {
+        string testText = "Sample text for testing";
+        LinkArea linkArea = new(7, 4);
+        _linkAreaUI.Start(linkArea);
+
+        _linkAreaUI.SampleText = testText;
+
+        _linkAreaUI.SampleText.Should().Be(testText);
+
+        dynamic testAccessor = _linkAreaUI.TestAccessor().Dynamic;
+        int selectionStart = (int)testAccessor._sampleEdit.SelectionStart;
+        int selectionLength = (int)testAccessor._sampleEdit.SelectionLength;
+
+        selectionStart.Should().Be(linkArea.Start);
+        selectionLength.Should().Be(linkArea.Length);
+    }
+
+    [Fact]
+    public void Value_SetGet_WorksCorrectly()
+    {
+        _linkAreaUI.Start(new object());
+        object? initialValue = _linkAreaUI.Value;
+
+        _linkAreaUI.End();
+        object? finalValue = _linkAreaUI.Value;
+
+        initialValue.Should().NotBeNull();
+        finalValue.Should().BeNull();
+    }
+
+    [Fact]
+    public void End_SetsValueToNull()
+    {
+        _linkAreaUI.Start(new LinkArea(1, 2));
+
+        _linkAreaUI.End();
+
+        _linkAreaUI.Value.Should().BeNull();
+    }
+
+    [Fact]
+    public void Start_SetsValueAndUpdatesSelection()
+    {
+        object? testValue = new LinkArea(3, 4);
+        _linkAreaUI.Start(testValue);
+
+        dynamic dynamicAccessor = _linkAreaUI.TestAccessor().Dynamic;
+        int selectionStart = (int)dynamicAccessor._sampleEdit.SelectionStart;
+        int selectionLength = (int)dynamicAccessor._sampleEdit.SelectionLength;
+
+        _linkAreaUI.Value.Should().Be(testValue);
+
+        string sampleText = _linkAreaUI.SampleText;
+        sampleText.Length.Should().BeGreaterThanOrEqualTo(selectionStart + selectionLength);
+
+        selectionStart.Should().Be(3);
+        selectionLength.Should().Be(4);
+    }
+
+    [Fact]
+    public void Start_WithNullValue_DoesNotThrow()
+    {
+        Action action = () => _linkAreaUI.Start(null);
+
+        action.Should().NotThrow();
+        _linkAreaUI.Value.Should().BeNull();
+    }
+}