Add PDF attachment and PDF/A conversion features

Introduced endpoints for embedding attachments in PDFs and converting
between standard PDFs and PDF/A formats. Added `PdfAttachmentController`
and `PdfConversionController` with multipart/form-data and Base64-based
support. Implemented commands, handlers, and validators for these
operations.

Extended `IPdfProcessor` with methods for adding attachments and
PDF/A conversion. Partially implemented functionality in
`DevExpressPdfProcessor`, including `ConvertFromPdfAAsync`.

Added `attachment.xml` and `withoutAttachment.pdf` as resources for
testing. Marked endpoints as `[Obsolete]` to indicate incomplete
implementation. Improved validation and error handling for commands.

DocumentOperator.Application/ConvertFromPdfA/ConvertFromPdfACommand.cs

@@ -0,0 +1,41 @@
+using DocumentOperator.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentOperator.Application.ConvertFromPdfA;
+
+/// <summary>
+/// Command to convert a PDF/A document to a standard PDF (removes PDF/A restrictions)
+/// </summary>
+public record ConvertFromPdfACommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF/A document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+}
+
+/// <summary>
+/// Handler for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ConvertFromPdfACommand, byte[]>
+{
+    public async Task<byte[]> Handle(ConvertFromPdfACommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ConvertFromPdfAAsync(request.PdfStream);
+    }
+}
+
+/// <summary>
+/// Validator for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandValidator : AbstractValidator<ConvertFromPdfACommand>
+{
+    public ConvertFromPdfACommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+    }
+}