CmdPal: Remove ShellViewModel's dependency on MainListPage (#40482)

targets #40479

Kinda mental that the viewmodel had this hard dependency.

So instead I added a service for providing the root page for the app.
Theoretically, you could add a different `IRootPageService`, and give
out
a different root page.


ref #40113

src/modules/cmdpal/Microsoft.CmdPal.UI/App.xaml.cs

@@ -144,6 +144,7 @@ public partial class App : Application
         services.AddSingleton<IExtensionService, ExtensionService>();
         services.AddSingleton<TrayIconService>();
 
+        services.AddSingleton<IRootPageService, PowerToysRootPageService>();
         services.AddSingleton(new TelemetryForwarder());
 
         // ViewModels

src/modules/cmdpal/Microsoft.CmdPal.UI/PowerToysRootPageService.cs

@@ -0,0 +1,71 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using ManagedCommon;
+using Microsoft.CmdPal.Common.Services;
+using Microsoft.CmdPal.UI.ViewModels;
+using Microsoft.CmdPal.UI.ViewModels.MainPage;
+using Microsoft.CommandPalette.Extensions;
+using Microsoft.Extensions.DependencyInjection;
+
+// To learn more about WinUI, the WinUI project structure,
+// and more about our project templates, see: http://aka.ms/winui-project-info.
+namespace Microsoft.CmdPal.UI;
+
+internal sealed class PowerToysRootPageService : IRootPageService
+{
+    private readonly IServiceProvider _serviceProvider;
+    private Lazy<MainListPage> _mainListPage;
+
+    public PowerToysRootPageService(IServiceProvider serviceProvider)
+    {
+        _serviceProvider = serviceProvider;
+
+        _mainListPage = new Lazy<MainListPage>(() =>
+        {
+            return new MainListPage(_serviceProvider);
+        });
+    }
+
+    public async Task PreLoadAsync()
+    {
+        var tlcManager = _serviceProvider.GetService<TopLevelCommandManager>()!;
+        await tlcManager.LoadBuiltinsAsync();
+    }
+
+    public Microsoft.CommandPalette.Extensions.IPage GetRootPage()
+    {
+        return _mainListPage.Value;
+    }
+
+    public async Task PostLoadRootPageAsync()
+    {
+        var tlcManager = _serviceProvider.GetService<TopLevelCommandManager>()!;
+
+        // After loading built-ins, and starting navigation, kick off a thread to load extensions.
+        tlcManager.LoadExtensionsCommand.Execute(null);
+
+        await tlcManager.LoadExtensionsCommand.ExecutionTask!;
+        if (tlcManager.LoadExtensionsCommand.ExecutionTask.Status != TaskStatus.RanToCompletion)
+        {
+            // TODO: Handle failure case
+        }
+    }
+
+    public void OnPerformTopLevelCommand(object? context)
+    {
+        try
+        {
+            if (context is IListItem listItem)
+            {
+                _mainListPage.Value.UpdateHistory(listItem);
+            }
+        }
+        catch (Exception ex)
+        {
+            Logger.LogError("Failed to update history in PowerToysRootPageService");
+            Logger.LogError(ex.ToString());
+        }
+    }
+}