Fix loading top-level commands (#40602)

## Summary of the Pull Request
Updates LoadTopLevelCommandsFromProvider so it returns list of top-level
commands for further instead of modifying TopLevelCommands collection
directly.

Reverts unintended change from cfa5f75 where
LoadTopLevelCommandsFromProvider updates the shared TopLevelCommands
collection directly from in a task, causing thread-safety issues and
bypassing synchronization lock on the collection.

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

@@ -104,14 +104,14 @@ public partial class TopLevelCommandManager : ObservableObject,
                 List<TopLevelViewModel> commands = [];
                 foreach (var item in commandProvider.TopLevelItems)
                 {
-                    TopLevelCommands.Add(item);
+                    commands.Add(item);
                 }
 
                 foreach (var item in commandProvider.FallbackItems)
                 {
                     if (item.IsEnabled)
                     {
-                        TopLevelCommands.Add(item);
+                        commands.Add(item);
                     }
                 }