CmdPal: Auto-focus first content (#39414)

This lets us auto-focus the first input on a `IFormContent`, if there's
only one `IContent` on the page.

This dramatically improves the usability for forms, since we'll
immediately put focus into them

Closes #38436

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

@@ -79,6 +79,9 @@ public partial class ContentPageViewModel : PageViewModel, ICommandBarContext
             throw;
         }
 
+        var oneContent = newContent.Count == 1;
+        newContent.ForEach(c => c.OnlyControlOnPage = oneContent);
+
         // Now, back to a UI thread to update the observable collection
         DoOnUiThread(
         () =>

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

@@ -7,4 +7,5 @@ namespace Microsoft.CmdPal.UI.ViewModels;
 public abstract partial class ContentViewModel(WeakReference<IPageContext> context) :
     ExtensionObjectViewModel(context)
 {
+    public bool OnlyControlOnPage { get; internal set; }
 }