add context menu back.

src/Avalonia.Themes.Default/TextBox.xaml

@@ -14,6 +14,7 @@
     <Setter Property="SelectionBrush" Value="{DynamicResource HighlightBrush}"/>
     <Setter Property="SelectionForegroundBrush" Value="{DynamicResource HighlightForegroundBrush}"/>
     <Setter Property="Padding" Value="4"/>
+    <Setter Property="ContextMenu" Value="{DynamicResource DefaultTextBoxContextMenu}" />
     <Setter Property="Template">
       <ControlTemplate>
         <Border Name="border"

src/Avalonia.Themes.Fluent/TextBox.xaml

@@ -33,6 +33,7 @@
     <Setter Property="MinWidth" Value="{DynamicResource TextControlThemeMinWidth}" />
     <Setter Property="Padding" Value="{DynamicResource TextControlThemePadding}" />
     <Setter Property="FocusAdorner" Value="{x:Null}" />
+    <Setter Property="ContextMenu" Value="{DynamicResource DefaultTextBoxContextMenu}" />
     <Setter Property="Template">
       <ControlTemplate>
         <DockPanel>

tests/Avalonia.LeakTests/ControlTests.cs

@@ -426,7 +426,7 @@ namespace Avalonia.LeakTests
         public void Attached_ContextMenu_Is_Freed()
         {
             using (Start())
-            {   
+            {
                 void AttachShowAndDetachContextMenu(Control control)
                 {
                     var contextMenu = new ContextMenu