Update Fallback commands async, once (#38157)

The problem: 

> * we need to go update all the Fallback commands. (these are ones that extensions can use to react to the search text - basically, "what the user typed wasn't found immediately, but here's something they can fall back on"
>   * this is wacky, because the way I had it, I update each item, and if it "changes visibility", then we need to update the main list, because we've already removed it from the list. So we need to re-update the list to account for that
>     * you missed it reading that (and i missed it writing it) but that basically means we re-populate the list F={num fallbacks} times, because each one sends the "do it again" message
>     * That results in us basically creating (F+1)*(N=num items+apps) view models, initializing them, and not needing most of them

The crux here being a single thread, to update all the fallback items,
that then only raises _one_ items changed at the very end.

I don't love this, one misbehaving fallback could stop all the others. In theory, we should do a parallel update of all these things, with a like, 1s timeout on each leg. 

But it has gotta be faster till we can do #38140 (or similar)

Closes: (not sure I filed one). But the first typed character _felt_ slow.

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

@@ -2,6 +2,7 @@
 // The Microsoft Corporation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Immutable;
 using System.Collections.Specialized;
 using CommunityToolkit.Mvvm.Messaging;
 using Microsoft.CmdPal.Ext.Apps;
@@ -101,12 +102,7 @@ public partial class MainListPage : DynamicListPage,
         var commands = _tlcManager.TopLevelCommands;
         lock (commands)
         {
-            // This gets called on a background thread, because ListViewModel
-            // updates the .SearchText of all extensions on a BG thread.
-            foreach (var command in commands)
-            {
-                command.TryUpdateFallbackText(newSearch);
-            }
+            UpdateFallbacks(newSearch, commands.ToImmutableArray());
 
             // Cleared out the filter text? easy. Reset _filteredItems, and bail out.
             if (string.IsNullOrEmpty(newSearch))
@@ -137,6 +133,26 @@ public partial class MainListPage : DynamicListPage,
         }
     }
 
+    private void UpdateFallbacks(string newSearch, IReadOnlyList<TopLevelViewModel> commands)
+    {
+        // fire and forget
+        _ = Task.Run(() =>
+        {
+            var needsToUpdate = false;
+
+            foreach (var command in commands)
+            {
+                var changedVisibility = command.SafeUpdateFallbackTextSynchronous(newSearch);
+                needsToUpdate = needsToUpdate || changedVisibility;
+            }
+
+            if (needsToUpdate)
+            {
+                RaiseItemsChanged();
+            }
+        });
+    }
+
     private bool ActuallyLoading()
     {
         var tlcManager = _serviceProvider.GetService<TopLevelCommandManager>()!;

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

@@ -4,8 +4,7 @@
 
 using System.Collections.ObjectModel;
 using CommunityToolkit.Mvvm.ComponentModel;
-using CommunityToolkit.Mvvm.Messaging;
-using Microsoft.CmdPal.UI.ViewModels.Messages;
+using ManagedCommon;
 using Microsoft.CmdPal.UI.ViewModels.Settings;
 using Microsoft.CommandPalette.Extensions;
 using Microsoft.CommandPalette.Extensions.Toolkit;
@@ -237,32 +236,46 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
         }
     }
 
-    public void TryUpdateFallbackText(string newQuery)
+    internal bool SafeUpdateFallbackTextSynchronous(string newQuery)
     {
         if (!IsFallback)
         {
-            return;
+            return false;
         }
 
-        _ = Task.Run(() =>
+        try
         {
-            try
-            {
-                var model = _commandItemViewModel.Model.Unsafe;
-                if (model is IFallbackCommandItem fallback)
-                {
-                    var wasEmpty = string.IsNullOrEmpty(Title);
-                    fallback.FallbackHandler.UpdateQuery(newQuery);
-                    var isEmpty = string.IsNullOrEmpty(Title);
-                    if (wasEmpty != isEmpty)
-                    {
-                        WeakReferenceMessenger.Default.Send<UpdateFallbackItemsMessage>();
-                    }
-                }
-            }
-            catch (Exception)
-            {
-            }
-        });
+            return UnsafeUpdateFallbackSynchronous(newQuery);
+        }
+        catch (Exception ex)
+        {
+            Logger.LogError(ex.ToString());
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Calls UpdateQuery on our command, if we're a fallback item. This does
+    /// RPC work, so make sure you're calling it on a BG thread.
+    /// </summary>
+    /// <param name="newQuery">The new search text to pass to the extension</param>
+    /// <returns>true if our Title changed across this call</returns>
+    private bool UnsafeUpdateFallbackSynchronous(string newQuery)
+    {
+        var model = _commandItemViewModel.Model.Unsafe;
+
+        // RPC to check type
+        if (model is IFallbackCommandItem fallback)
+        {
+            var wasEmpty = string.IsNullOrEmpty(Title);
+
+            // RPC for method
+            fallback.FallbackHandler.UpdateQuery(newQuery);
+            var isEmpty = string.IsNullOrEmpty(Title);
+            return wasEmpty != isEmpty;
+        }
+
+        return false;
     }
 }