CmdPal: Sync access to TopLevelCommandManager from UpdateCommandsForProvider (#40752)

## Summary of the Pull Request

Fixes unsynchronized access to `LoadTopLevelCommands` in
`TopLevelCommandManager.UpdateCommandsForProvider`, which previously led
to `InvalidOperationException: Collection was modified`.

Addressing this also uncovered another issue: overlapping invocations of
`ReloadAllCommandsAsync` were causing duplication of items in the main
list -- so I'm fixing that as well.

## PR Checklist

- [x] Closes
    - Fixes #38194 
    - Partially solves #40776
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:**
- [x] **Localization:** none
- [x] **Dev docs:** none
- [x] **New binaries:** nope
- [x] **Documentation updated:** no need

## Detailed Description of the Pull Request / Additional comments

## Validation Steps Performed
Tested with bookmarks.

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

@@ -47,6 +47,8 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
 
     public CommandItemViewModel ItemViewModel => _commandItemViewModel;
 
+    public string CommandProviderId => _commandProviderId;
+
     ////// ICommandItem
     public string Title => _commandItemViewModel.Title;
 
@@ -351,4 +353,9 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
     {
         return new PerformCommandMessage(this.CommandViewModel.Model, new Core.ViewModels.Models.ExtensionObject<IListItem>(this));
     }
+
+    public override string ToString()
+    {
+        return $"{nameof(TopLevelViewModel)}: {Id} ({Title}) - display: {DisplayTitle} - fallback: {IsFallback} - enabled: {IsEnabled}";
+    }
 }