Fully initialize context menus when they change (#38998)

If we don't slow-initialize the whole menu when it changes, then we won't see that there's secondary (& more) commands.  

Tested this with the extension from [waaverecords/CmdPal.Ext.Spotify#4](https://github.com/waaverecords/CmdPal.Ext.Spotify/pull/4)

Closes #38959 


Also seemingly closes #38347 - seems that needed additional bumping of the `EmptyContent`'s

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

@@ -29,6 +29,8 @@ public partial class CommandBarViewModel : ObservableObject,
 
             field = value;
             SetSelectedItem(value);
+
+            OnPropertyChanged(nameof(SelectedItem));
         }
     }
 
@@ -115,6 +117,10 @@ public partial class CommandBarViewModel : ObservableObject,
         {
             ShouldShowContextMenu = false;
         }
+
+        OnPropertyChanged(nameof(HasSecondaryCommand));
+        OnPropertyChanged(nameof(SecondaryCommand));
+        OnPropertyChanged(nameof(ShouldShowContextMenu));
     }
 
     // InvokeItemCommand is what this will be in Xaml due to source generator