Potential fix for pull request finding 'Useless assignment to local variable'

Co-authored-by: Copilot Autofix powered by AI <223894421+github-code-quality[bot]@users.noreply.github.com>

tests/ImageSharp.Tests/Formats/Bmp/BmpDecoderTests.cs

@@ -589,7 +589,7 @@ public class BmpDecoderTests
 
         using MemoryStream stream = new(bmp);
 
-        InvalidImageContentException ex = Assert.Throws<InvalidImageContentException>(() =>
+        Assert.Throws<InvalidImageContentException>(() =>
         {
             using Image image = BmpDecoder.Instance.Decode(DecoderOptions.Default, stream);
         });