Added a UI Test (#18668)

src/Controls/tests/TestCases.HostApp/Issues/Issue18668.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="Maui.Controls.Sample.Issues.Issue18668">
+    <VerticalStackLayout>
+        <RadioButton x:Name="radioButton" Content="Hello, Maui!"/>
+        <Button AutomationId="button" Text="Disable radio button" Clicked="ButtonClicked"/>
+    </VerticalStackLayout>
+</ContentPage>

src/Controls/tests/TestCases.HostApp/Issues/Issue18668.xaml.cs

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+namespace Maui.Controls.Sample.Issues
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	[Issue(IssueTracker.Github, 18668, "Visual state change for disabled RadioButton", PlatformAffected.All)]
+	public partial class Issue18668 : ContentPage
+	{
+		public Issue18668()
+		{
+			InitializeComponent();
+		}
+
+		private void ButtonClicked(object sender, EventArgs e)
+		{
+			radioButton.IsEnabled = false;
+		}
+	}
+}

src/Controls/tests/TestCases.Shared.Tests/Tests/Issues/Issue18668.cs

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+using NUnit.Framework;
+using UITest.Appium;
+using UITest.Core;
+
+namespace Microsoft.Maui.TestCases.Tests.Issues
+{
+	public class Issue18668 : _IssuesUITest
+	{
+		public override string Issue => "Visual state change for disabled RadioButton";
+
+		public Issue18668(TestDevice device) : base(device){ }
+
+		[Test]
+		[Category(UITestCategories.RadioButton)]
+		public void TestIssue18668()
+		{
+			App.WaitForElement("button");
+			App.Click("button");
+
+			// The test passes if the radio button is visually disabled
+			VerifyScreenshot();
+		}
+	}
+}