Add Content attribute to ContentPresenter (#21997)

* Add Content attribute to ContentPresenter

* Add ContentPresenter default content property test

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -364,6 +364,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets the content to be displayed by the presenter.
         /// </summary>
+        [Content]
         [DependsOn(nameof(ContentTemplate))]
         public object? Content
         {

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/BasicTests.cs

@@ -42,6 +42,17 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             Assert.Equal("Foo", target.Content);
         }
 
+        [Fact]
+        public void ContentPresenter_Default_Content_Property_Is_Set()
+        {
+            var xaml = @"<ContentPresenter xmlns='https://github.com/avaloniaui'>Foo</ContentPresenter>";
+
+            var target = AvaloniaRuntimeXamlLoader.Parse<ContentPresenter>(xaml);
+
+            Assert.NotNull(target);
+            Assert.Equal("Foo", target.Content);
+        }
+
         [Fact]
         public void Attached_Property_Is_Set()
         {