refactor(configmanager): rename UI project and split test projects

Rename ConfigManager to ConfigManager.Ui to match the Core/CLI/UI project
structure, and split the monolithic test project into Core.Tests,
Cli.Tests, and Ui.Tests to align with the source project organization.

NEW/tests/Utils/JdeScoping.ConfigManager.Ui.Tests/TestAppBuilder.cs

@@ -0,0 +1,21 @@
+using Avalonia;
+using Avalonia.Headless;
+using JdeScoping.ConfigManager.Ui;
+
+[assembly: AvaloniaTestApplication(typeof(JdeScoping.ConfigManager.Ui.Tests.TestAppBuilder))]
+
+namespace JdeScoping.ConfigManager.Ui.Tests;
+
+/// <summary>
+/// Provides a headless Avalonia application builder for UI tests.
+/// </summary>
+public class TestAppBuilder
+{
+    /// <summary>
+    /// Builds the Avalonia application configured for headless testing.
+    /// </summary>
+    /// <returns>An <see cref="AppBuilder"/> configured with headless platform options.</returns>
+    public static AppBuilder BuildAvaloniaApp() =>
+        AppBuilder.Configure<App>()
+            .UseHeadless(new AvaloniaHeadlessPlatformOptions());
+}