CmdPal: Cancel page load when superseded by a new page navigation (#42233)

## Summary of the Pull Request

This PR introduces cancellation support for navigation. If a user
navigates to page X and then returns back or navigates elsewhere before
the page X fully loads, this update ensures that page X will not set
itself as the current page and is ignored.

It resolves the issue where returning to the home page left the previous
page's icon and placeholder visible in the search bar, causing the
search functionality to fail.

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## PR Checklist

- [x] Closes: #42247
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Pages/ShellPage.xaml.cs

@@ -95,7 +95,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         AddHandler(KeyDownEvent, new KeyEventHandler(ShellPage_OnKeyDown), false);
         AddHandler(PointerPressedEvent, new PointerEventHandler(ShellPage_OnPointerPressed), true);
 
-        RootFrame.Navigate(typeof(LoadingPage), ViewModel);
+        RootFrame.Navigate(typeof(LoadingPage), new AsyncNavigationRequest(ViewModel, CancellationToken.None));
 
         var pageAnnouncementFormat = ResourceLoaderInstance.GetString("ScreenReader_Announcement_NavigatedToPage0");
         _pageNavigatedAnnouncement = CompositeFormat.Parse(pageAnnouncementFormat);
@@ -153,7 +153,7 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
                     ContentPageViewModel => typeof(ContentPage),
                     _ => throw new NotSupportedException(),
                 },
-                message.Page,
+                new AsyncNavigationRequest(message.Page, message.CancellationToken),
                 message.WithAnimation ? DefaultPageAnimation : _noAnimation);
 
             PowerToysTelemetry.Log.WriteEvent(new OpenPage(RootFrame.BackStackDepth));
@@ -403,6 +403,8 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
     {
         HideDetails();
 
+        ViewModel.CancelNavigation();
+
         // Note: That we restore the VM state below in RootFrame_Navigated call back after this occurs.
         // In the future, we may want to manage the back stack ourselves vs. relying on Frame
         // We could replace Frame with a ContentPresenter, but then have to manage transition animations ourselves.
@@ -456,11 +458,32 @@ public sealed partial class ShellPage : Microsoft.UI.Xaml.Controls.Page,
         // This listens to the root frame to ensure that we also track the content's page VM as well that we passed as a parameter.
         // This is currently used for both forward and backward navigation.
         // As when we go back that we restore ourselves to the proper state within our VM
-        if (e.Parameter is PageViewModel page)
+        if (e.Parameter is AsyncNavigationRequest request)
         {
-            // Note, this shortcuts and fights a bit with our LoadPageViewModel above, but we want to better fast display and incrementally load anyway
-            // We just need to reconcile our loading systems a bit more in the future.
-            ViewModel.CurrentPage = page;
+            if (request.NavigationToken.IsCancellationRequested && e.NavigationMode is not (Microsoft.UI.Xaml.Navigation.NavigationMode.Back or Microsoft.UI.Xaml.Navigation.NavigationMode.Forward))
+            {
+                return;
+            }
+
+            switch (request.TargetViewModel)
+            {
+                case PageViewModel pageViewModel:
+                    ViewModel.CurrentPage = pageViewModel;
+                    break;
+                case ShellViewModel:
+                    // This one is an exception, for now (LoadingPage is tied to ShellViewModel,
+                    // but ShellViewModel is not PageViewModel.
+                    ViewModel.CurrentPage = ViewModel.NullPage;
+                    break;
+                default:
+                    ViewModel.CurrentPage = ViewModel.NullPage;
+                    Logger.LogWarning($"Invalid navigation target: AsyncNavigationRequest.{nameof(AsyncNavigationRequest.TargetViewModel)} must be {nameof(PageViewModel)}");
+                    break;
+            }
+        }
+        else
+        {
+            Logger.LogWarning("Unrecognized target for shell navigation: " + e.Parameter);
         }
 
         if (e.Content is Page element)