refactor(BurnPdfCommand): replace manual JSON serialization with ToJson extension

- Replaced `System.Text.Json.JsonSerializer.Serialize(request, Format.Json.ForDiagnostics)`
  with `request.ToJson(Format.Json.ForDiagnostics)` for better readability and consistency.
- No functional changes, purely a code style and maintainability improvement.

EnvelopeGenerator.Application/Pdf/BurnPdfCommand.cs

@@ -71,15 +71,15 @@ public class BurnPdfCommandHandler : IRequestHandler<BurnPdfCommand, byte[]>
             .Include(env => env.Documents!).ThenInclude(doc => doc.Elements!).ThenInclude(element => element.Annotations)
             .FirstOrDefaultAsync(cancel)
             ?? throw new BadRequestException($"Envelope could not be found. Request details:\n" +
-                System.Text.Json.JsonSerializer.Serialize(request, Format.Json.ForDiagnostics));
+                request.ToJson(Format.Json.ForDiagnostics));
 
         var doc = envelope.Documents?.FirstOrDefault()
             ?? throw new NotFoundException($"Document could not be located within the specified envelope. Request details:\n" +
-                System.Text.Json.JsonSerializer.Serialize(request, Format.Json.ForDiagnostics));
+                request.ToJson(Format.Json.ForDiagnostics));
 
         if (doc.ByteData is null)
             throw new InvalidOperationException($"Document byte data is missing, indicating a potential data integrity issue. Request details:\n" +
-                System.Text.Json.JsonSerializer.Serialize(request, Format.Json.ForDiagnostics));
+                request.ToJson(Format.Json.ForDiagnostics));
 
         return doc.Elements?.SelectMany(e => e.Annotations ?? Enumerable.Empty<ElementAnnotation>()).Where(annot => annot is not null).Any() ?? false
             ? BurnElementAnnotsToPDF(doc.ByteData, doc.Elements)