Add unit test for LinkAreaEditor (#12718)

Related #10773

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

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/LinkAreaEditorTests.cs

@@ -0,0 +1,52 @@
+// 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;
+using static System.Windows.Forms.Design.LinkAreaEditor;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public sealed class LinkAreaEditorTests
+{
+    [Fact]
+    public void EditValue_ShouldBehaveCorrectly_BasedOnEditorServiceAvailability()
+    {
+        LinkAreaEditor editor = new();
+        Mock<ITypeDescriptorContext> context = new();
+        Mock<IServiceProvider> provider = new();
+        Mock<IWindowsFormsEditorService> editorService = new();
+        LinkArea value = new(0, 5);
+        using Label instance = new() { Text = "Initial Text" };
+        var property = TypeDescriptor.GetProperties(instance)["Text"];
+
+        provider.Setup(p => p.GetService(typeof(IWindowsFormsEditorService))).Returns((object?)null);
+        editor.EditValue(context.Object, provider.Object, value).Should().Be(value);
+
+        provider.Setup(p => p.GetService(typeof(IWindowsFormsEditorService))).Returns(editorService.Object);
+        editorService.Setup(es => es.ShowDialog(It.IsAny<Form>())).Returns(DialogResult.OK);
+        var result = editor.EditValue(context.Object, provider.Object, value);
+        result.Should().BeOfType<LinkArea>();
+        if (result is LinkArea linkAreaResult)
+        {
+            linkAreaResult.Length.Should().Be(value.Length);
+        }
+
+        property?.SetValue(instance, "Updated Text");
+        context.Setup(c => c.Instance).Returns(instance);
+        editor.EditValue(context.Object, provider.Object, value).Should().NotBeNull();
+        property?.GetValue(instance).Should().Be("Updated Text");
+
+        editorService.Setup(es => es.ShowDialog(It.IsAny<Form>())).Callback<Form>(form =>
+        {
+            if (form is LinkAreaUI linkAreaUI)
+            {
+                linkAreaUI.SampleText = "Updated Text2";
+            }
+        }).Returns(DialogResult.OK);
+        editor.EditValue(context.Object, provider.Object, value).Should().NotBeNull();
+        property?.GetValue(instance).Should().Be("Updated Text2");
+    }
+}