Fix CmdPal apps extension ignoring the fallback results limit setting (#45716)

## Summary

MainListPage hardcoded _appResultLimit = 10 instead of reading from
AllAppsCommandProvider.TopLevelResultLimit, which correctly parses the
user's SearchResultLimit setting. This meant changing the results limit
in settings had no effect on the apps extension fallback results.

##  Changes

- `MainListPage.cs` — Replaced the hardcoded _appResultLimit = 10 field
with a computed property AppResultLimit that delegates to
AllAppsCommandProvider.TopLevelResultLimit.
- `MainListPageResultFactoryTests.cs` — Added three regression tests:
- `Merge_AppLimitOfOne_ReturnsOnlyTopApp` — verifies limit of 1 returns
only the top app
- `Merge_AppLimitOfZero_ReturnsNoApps` — verifies limit of 0 returns no
apps
- `Merge_AppLimitOfOne_WithOtherResults_AppsAreLimited` — verifies apps
are limited even when mixed with other result types

## Validation

- [X]  Existing tests pass
- [X] New regression tests cover edge cases for the appResultLimit
parameter

## Linked Issues

- Fixes #45654

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/MainListPage.cs

@@ -47,7 +47,8 @@ public sealed partial class MainListPage : DynamicListPage,
 
     private bool _includeApps;
     private bool _filteredItemsIncludesApps;
-    private int _appResultLimit = 10;
+
+    private int AppResultLimit => AllAppsCommandProvider.TopLevelResultLimit;
 
     private InterlockedBoolean _refreshRunning;
     private InterlockedBoolean _refreshRequested;
@@ -190,7 +191,7 @@ public sealed partial class MainListPage : DynamicListPage,
                     validScoredFallbacks,
                     _filteredApps,
                     validFallbacks,
-                    _appResultLimit);
+                    AppResultLimit);
             }
         }
     }