BugFix: BasemapGallery fails to load default Basemaps in WPF (#586)

* Optimize BasemapGallery initialization

Removed redundant initialization of `AvailableBasemaps` in the `BasemapGallery` constructor. Added a null check in `BasemapGallery_Loaded` to initialize `AvailableBasemaps` only if it is null, calling `_controller.LoadFromDefaultPortal()` asynchronously if needed. This change prevents unnecessary initialization and allows for external setting of `AvailableBasemaps` before the control is loaded.

* Initialize AvailableBasemaps and improve load check

Add initialization flag for BasemapGallery collection

src/Toolkit/Toolkit.UI.Controls/BasemapGallery/BasemapGallery.Windows.cs

@@ -34,6 +34,7 @@ namespace Esri.ArcGISRuntime.Toolkit.UI.Controls
     public partial class BasemapGallery : Control
     {
         private readonly BasemapGalleryController _controller;
+        private bool isAvailableBasemapCollectionInitialized;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="BasemapGallery"/> class.
@@ -44,6 +45,7 @@ public BasemapGallery()
             DefaultStyleKey = typeof(BasemapGallery);
             SizeChanged += BasemapGallerySizeChanged;
             AvailableBasemaps = new ObservableCollection<BasemapGalleryItem>();
+            isAvailableBasemapCollectionInitialized = true;
             _controller.PropertyChanged += HandleControllerPropertyChanged;
             Loaded += BasemapGallery_Loaded;
         }
@@ -52,8 +54,8 @@ private async void BasemapGallery_Loaded(object? sender, RoutedEventArgs e)
         {
             // Unsubscribe from the Loaded event to ensure this only runs once.
             Loaded -= BasemapGallery_Loaded;
-            
-            if (AvailableBasemaps is null)
+
+            if ((AvailableBasemaps is null || !AvailableBasemaps.Any()) && isAvailableBasemapCollectionInitialized)
             {
                 await _controller.LoadFromDefaultPortal();
             }
@@ -141,6 +143,7 @@ private static void AvailableBasemapsChanged(DependencyObject d, DependencyPrope
         {
             if (d is BasemapGallery gallery)
             {
+                gallery.isAvailableBasemapCollectionInitialized = false;
                 if (e.NewValue != gallery._controller.AvailableBasemaps)
                 {
                     gallery._controller.AvailableBasemaps = e.NewValue as IList<BasemapGalleryItem>;