test: fix timestamp preservation for Linux compatibility

Only assert creation time on Windows as Linux birthtime support is
inconsistent and was causing CI failures.

Author: liphvf

FurLab.Tests/Commands/EncodingConversionServiceTests.cs

@@ -400,7 +400,10 @@ public async Task ConvertFiles_PreservesTimestamps()
         await _service.ConvertFilesAsync(options);
 
         // Verify timestamps are preserved
-        Assert.AreEqual(creationTime, File.GetCreationTime(filePath));
+        if (OperatingSystem.IsWindows())
+        {
+            Assert.AreEqual(creationTime, File.GetCreationTime(filePath));
+        }
         Assert.AreEqual(lastWriteTime, File.GetLastWriteTime(filePath));
     }