[Android] Fixed selection logic with header/footer in CollectionView

Adjusted SelectableItemsViewAdapter to correctly handle selection when header or footer templates are present. Updated Issue21375 test case to include header and footer templates.

Author: kubaflo

src/Controls/src/Core/Handlers/Items/Android/Adapters/SelectableItemsViewAdapter.cs

@@ -99,10 +99,10 @@ internal void MarkPlatformSelection(SelectableItemsView selectableItemsView)
 					return;
 			}
 
-			for (int i = 0; i < _currentViewHolders.Count; i++)
+			for (int i = 0; i < _currentViewHolders.Count - (ItemsSource.HasFooter ? 1 : 0); i++)
 			{
 				var holder = _currentViewHolders[i];
-				if (holder.BindingAdapterPosition >= 0)
+				if (holder.BindingAdapterPosition >= (ItemsSource.HasHeader ? 1 : 0))
 				{
 					var item = ItemsSource.GetItem(holder.BindingAdapterPosition);
 					bool shouldBeSelected = _selectedSet.Contains(item);

src/Controls/tests/TestCases.HostApp/Issues/Issue21375.xaml

@@ -5,6 +5,16 @@
              Title="Issue21375">
     <VerticalStackLayout>
         <CollectionView ItemsSource="{Binding Items}" SelectionMode="Single" x:Name="collectionView" AutomationId="collectionView" Background="LightBlue">
+        <CollectionView.HeaderTemplate>
+            <DataTemplate>
+                <Label HeightRequest="0" Text="HeaderTemplate"/>
+            </DataTemplate>
+        </CollectionView.HeaderTemplate>
+        <CollectionView.FooterTemplate>
+            <DataTemplate>
+                <Label HeightRequest="0" Text="FooterTemplate"/>
+            </DataTemplate>
+        </CollectionView.FooterTemplate>
             <CollectionView.ItemTemplate>
                 <DataTemplate>
                     <StackLayout Padding="10" SemanticProperties.Description="{Binding Name}" HeightRequest="50">