fix: ensure proper StringBuilderCache release (#18985)

src/Avalonia.Controls/TextBox.cs

@@ -2183,7 +2183,7 @@ namespace Avalonia.Controls
                 textBuilder.Append(text);
                 textBuilder.Remove(start, end - start);
 
-                SetCurrentValue(TextProperty, textBuilder.ToString());
+                SetCurrentValue(TextProperty, StringBuilderCache.GetStringAndRelease(textBuilder));
 
                 _presenter?.MoveCaretToTextPosition(start);