CmdPal: Actually observe Details

Extensions can change the properties on their Details, and they should
be observable, but they weren't. This is because the ShellPage is
ultimately responsible for exposing the details, but it doesn't own the
details. The selected ListItemViewModel from the ListPage does.

This PR just adds a event handler on ListViewModel. We'll attach/detach
that handler to ListItemViewModels as the selection changes. In the body
of that handler, we'll let the ShellPage know when the details object
changes (by sending ShowDetails/HideDetails messages).

Closes #39216

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

@@ -61,6 +61,8 @@ public partial class ListViewModel : PageViewModel, IDisposable
     private Task? _initializeItemsTask;
     private CancellationTokenSource? _cancellationTokenSource;
 
+    private ListItemViewModel? _lastSelectedItem;
+
     public override bool IsInitialized
     {
         get => base.IsInitialized; protected set
@@ -330,6 +332,11 @@ public partial class ListViewModel : PageViewModel, IDisposable
     [RelayCommand]
     private void UpdateSelectedItem(ListItemViewModel? item)
     {
+        if (_lastSelectedItem != null)
+        {
+            _lastSelectedItem.PropertyChanged -= SelectedItemPropertyChanged;
+        }
+
         if (item != null)
         {
             SetSelectedItem(item);
@@ -367,6 +374,43 @@ public partial class ListViewModel : PageViewModel, IDisposable
 
                TextToSuggest = item.TextToSuggest;
            });
+
+        _lastSelectedItem = item;
+        _lastSelectedItem.PropertyChanged += SelectedItemPropertyChanged;
+    }
+
+    private void SelectedItemPropertyChanged(object? sender, System.ComponentModel.PropertyChangedEventArgs e)
+    {
+        var item = _lastSelectedItem;
+        if (item == null)
+        {
+            return;
+        }
+
+        // already on the UI thread here
+        switch (e.PropertyName)
+        {
+            case nameof(item.Command):
+            case nameof(item.SecondaryCommand):
+            case nameof(item.AllCommands):
+            case nameof(item.Name):
+                WeakReferenceMessenger.Default.Send<UpdateCommandBarMessage>(new(item));
+                break;
+            case nameof(item.Details):
+                if (ShowDetails && item.HasDetails)
+                {
+                    WeakReferenceMessenger.Default.Send<ShowDetailsMessage>(new(item.Details));
+                }
+                else
+                {
+                    WeakReferenceMessenger.Default.Send<HideDetailsMessage>();
+                }
+
+                break;
+            case nameof(item.TextToSuggest):
+                TextToSuggest = item.TextToSuggest;
+                break;
+        }
     }
 
     private void ClearSelectedItem()