CmdPal: Cancel page load when superseded by a new page navigation (#42233)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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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

src/modules/cmdpal/ext/SamplePagesExtension/SamplesListPage.cs

@@ -48,6 +48,11 @@ public partial class SamplesListPage : ListPage
             Title = "Sample Icon Page",
             Subtitle = "A demo of using icons in various ways",
         },
+        new ListItem(new SlowListPage())
+        {
+            Title = "Slow loading list page",
+            Subtitle = "A demo of a list page that takes a while to load",
+        },
 
         // Content pages
         new ListItem(new SampleContentPage())