Fix DFF round-trip test to use ID3v2 builder

The MinimalFile_PreservesStructure test was using CreateWithMetadata
(DITI/DIAR native chunks) but asserting on Id3v2Tag. Since DFF Render()
only writes ID3v2 tags, use CreateWithId3v2 for proper round-trip testing.

Author: decriptor

tests/TagLibSharp2.Tests/Dff/DffFileRoundTripTests.cs

@@ -17,8 +17,8 @@ public class DffFileRoundTripTests
 	[TestMethod]
 	public void MinimalFile_PreservesStructure ()
 	{
-		// Arrange
-		var original = TestBuilders.Dff.CreateWithMetadata (title: "Test Title", artist: "Test Artist");
+		// Arrange - use CreateWithId3v2 since Render() only preserves ID3v2 tags
+		var original = TestBuilders.Dff.CreateWithId3v2 (title: "Test Title", artist: "Test Artist");
 
 		// Act
 		var result1 = DffFile.Read (original);