CmdPal: Update special fallbacks separately from the other fallbacks (#42289)

## Summary of the Pull Request

This PR introduces a hotfix that updates special fallback items
separately from the rest. This allows the loop handling special fallback
items to finish faster, ensuring they are not delayed by other fallback
items. As a result, calculator and run fallback items will be more
readily available to users.

This partially solves #42286 for special fallback items.

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## PR Checklist

- [ ] Related to: #42286
- [ ] **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.ViewModels/Commands/MainListPage.cs

@@ -244,7 +244,36 @@ public partial class MainListPage : DynamicListPage,
         var commands = _tlcManager.TopLevelCommands;
         lock (commands)
         {
-            UpdateFallbacks(SearchText, commands.ToImmutableArray(), token);
+            if (token.IsCancellationRequested)
+            {
+                return;
+            }
+
+            // prefilter fallbacks
+            var specialFallbacks = new List<TopLevelViewModel>(_specialFallbacks.Length);
+            var commonFallbacks = new List<TopLevelViewModel>();
+
+            foreach (var s in commands)
+            {
+                if (!s.IsFallback)
+                {
+                    continue;
+                }
+
+                if (_specialFallbacks.Contains(s.CommandProviderId))
+                {
+                    specialFallbacks.Add(s);
+                }
+                else
+                {
+                    commonFallbacks.Add(s);
+                }
+            }
+
+            // start update of fallbacks; update special fallbacks separately,
+            // so they can finish faster
+            UpdateFallbacks(SearchText, specialFallbacks, token);
+            UpdateFallbacks(SearchText, commonFallbacks, token);
 
             if (token.IsCancellationRequested)
             {
@@ -316,15 +345,12 @@ public partial class MainListPage : DynamicListPage,
             // with a list of all our commands & apps.
             if (!newFilteredItems.Any() && !newApps.Any())
             {
-                // We're going to start over with our fallbacks
-                newFallbacks = Enumerable.Empty<IListItem>();
-
                 newFilteredItems = commands.Where(s => !s.IsFallback);
 
                 // Fallbacks are always included in the list, even if they
                 // don't match the search text. But we don't want to
                 // consider them when filtering the list.
-                newFallbacks = commands.Where(s => s.IsFallback && !_specialFallbacks.Contains(s.CommandProviderId));
+                newFallbacks = commonFallbacks;
 
                 if (token.IsCancellationRequested)
                 {