Prevent apps from appearing in top-level search when Installed apps extension is disabled (#40132)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
Prevents installed applications from appearing in the top-level search
when Installed Apps extension is disabled.

Previously, application commands were still returned in the global
search results even when the *Installed Apps* extension was turned off.
To match user expectations, the search now respects the extension’s
enabled state.


- Added `IsActive` property to `CommandProviderWrapper` to indicate
whether the provider is both valid and enabled by the user in the
settings.
- Updated `MainListPage` to verify that the provider for `AllApps` is
active before including apps in filtered results.

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [x] **Closes:** #39937
- [x] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [x] **Tests:** Added/updated and all pass
- [x] **Localization:** All end-user-facing strings can be localized
- [x] **Dev docs:** nothing to update
- [x] **New binaries:** none
- [x] **Documentation updated:** nothing to update

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
Verified that the Installed app entries are shown in the top-level
search only when the Installed apps extension is enabled. Verified that
turning the Installed apps extension on or off has an immediate effect,
and that the behavior persists after an application restart.

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/TopLevelCommandManager.cs

@@ -26,6 +26,7 @@ public partial class TopLevelCommandManager : ObservableObject,
 
     private readonly List<CommandProviderWrapper> _builtInCommands = [];
     private readonly List<CommandProviderWrapper> _extensionCommandProviders = [];
+    private readonly Lock _commandProvidersLock = new();
 
     TaskScheduler IPageContext.Scheduler => _taskScheduler;
 
@@ -41,14 +42,26 @@ public partial class TopLevelCommandManager : ObservableObject,
     [ObservableProperty]
     public partial bool IsLoading { get; private set; } = true;
 
-    public IEnumerable<CommandProviderWrapper> CommandProviders => _builtInCommands.Concat(_extensionCommandProviders);
+    public IEnumerable<CommandProviderWrapper> CommandProviders
+    {
+        get
+        {
+            lock (_commandProvidersLock)
+            {
+                return _builtInCommands.Concat(_extensionCommandProviders).ToList();
+            }
+        }
+    }
 
     public async Task<bool> LoadBuiltinsAsync()
     {
         var s = new Stopwatch();
         s.Start();
 
-        _builtInCommands.Clear();
+        lock (_commandProvidersLock)
+        {
+            _builtInCommands.Clear();
+        }
 
         // Load built-In commands first. These are all in-proc, and
         // owned by our ServiceProvider.
@@ -56,7 +69,11 @@ public partial class TopLevelCommandManager : ObservableObject,
         foreach (var provider in builtInCommands)
         {
             CommandProviderWrapper wrapper = new(provider, _taskScheduler);
-            _builtInCommands.Add(wrapper);
+            lock (_commandProvidersLock)
+            {
+                _builtInCommands.Add(wrapper);
+            }
+
             var commands = await LoadTopLevelCommandsFromProvider(wrapper);
             lock (TopLevelCommands)
             {
@@ -185,6 +202,7 @@ public partial class TopLevelCommandManager : ObservableObject,
         IsLoading = true;
         var extensionService = _serviceProvider.GetService<IExtensionService>()!;
         await extensionService.SignalStopExtensionsAsync();
+
         lock (TopLevelCommands)
         {
             TopLevelCommands.Clear();
@@ -210,7 +228,11 @@ public partial class TopLevelCommandManager : ObservableObject,
         extensionService.OnExtensionRemoved -= ExtensionService_OnExtensionRemoved;
 
         var extensions = (await extensionService.GetInstalledExtensionsAsync()).ToImmutableList();
-        _extensionCommandProviders.Clear();
+        lock (_commandProvidersLock)
+        {
+            _extensionCommandProviders.Clear();
+        }
+
         if (extensions != null)
         {
             await StartExtensionsAndGetCommands(extensions);
@@ -247,9 +269,9 @@ public partial class TopLevelCommandManager : ObservableObject,
         // Wait for all extensions to start
         var wrappers = (await Task.WhenAll(startTasks)).Where(wrapper => wrapper != null).Select(w => w!).ToList();
 
-        foreach (var wrapper in wrappers)
+        lock (_commandProvidersLock)
         {
-            _extensionCommandProviders.Add(wrapper!);
+            _extensionCommandProviders.AddRange(wrappers);
         }
 
         // Load the commands from the providers in parallel
@@ -375,4 +397,13 @@ public partial class TopLevelCommandManager : ObservableObject,
         var errorMessage = $"A bug occurred in {$"the \"{extensionHint}\"" ?? "an unknown's"} extension's code:\n{ex.Message}\n{ex.Source}\n{ex.StackTrace}\n\n";
         CommandPaletteHost.Instance.Log(errorMessage);
     }
+
+    internal bool IsProviderActive(string id)
+    {
+        lock (_commandProvidersLock)
+        {
+            return _builtInCommands.Any(wrapper => wrapper.Id == id && wrapper.IsActive)
+                   || _extensionCommandProviders.Any(wrapper => wrapper.Id == id && wrapper.IsActive);
+        }
+    }
 }