Tests: WriteSetTests reads the byte string through System.Buffers' ToArray, which the second fix lacked the using for

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/GameCult.Caching.Tests/WriteSetTests.cs

@@ -1,5 +1,6 @@
 #nullable enable
 using System;
+using System.Buffers;
 using System.Collections.Generic;
 using System.Globalization;
 using System.IO;