fix(pdf): Replace XFontStyleEx with XFontStyle enum

- Update font style parameter from XFontStyleEx.Regular to XFontStyle.Regular
- Resolve API compatibility issue in PdfGenerationService
- Ensure correct enum type usage for PDF font styling

server/MiAssessment/src/MiAssessment.Core/Services/PdfGenerationService.cs

@@ -410,7 +410,7 @@ public class PdfGenerationService : IPdfGenerationService
     {
         using var document = new PdfDocument();
         var totalPages = images.Count;
-        var font = new XFont("Arial", 10, XFontStyleEx.Regular);
+        var font = new XFont("Arial", 10, XFontStyle.Regular);
         var brush = new XSolidBrush(XColor.FromArgb(153, 153, 153)); // #999999
 
         for (var i = 0; i < totalPages; i++)