Apply ConditionalClass attribute and simplify MarkItDownReaderTests

- Remove blank line between System and other using directives
- Add [ConditionalClass] attribute to MarkItDownReaderTests class
- Simplify CreateDocumentReader method to directly return reader without conditional check
- ConditionalClass handles test skipping at class level, eliminating need for manual condition checks

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Author: Copilot

test/Libraries/Microsoft.Extensions.DataIngestion.Tests/Readers/MarkItDownReaderTests.cs

@@ -4,18 +4,16 @@
 using System;
 using System.Linq;
 using System.Threading.Tasks;
-
 using Microsoft.DotNet.XUnitExtensions;
 using Xunit;
 
 namespace Microsoft.Extensions.DataIngestion.Readers.Tests;
 
+[ConditionalClass(typeof(MarkItDownCondition), nameof(MarkItDownCondition.IsMarkItDownInstalled))]
 public class MarkItDownReaderTests : DocumentReaderConformanceTests
 {
     protected override IngestionDocumentReader CreateDocumentReader(bool extractImages = false)
-        => MarkItDownCondition.IsInstalled.Value
-        ? new MarkItDownReader(extractImages: extractImages)
-        : throw new SkipTestException("MarkItDown is not installed");
+        => new MarkItDownReader(extractImages: extractImages);
 
     protected override void SimpleAsserts(IngestionDocument document, string source, string expectedId)
     {