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.Infrastructure/Services/PdfProcessing/DevExpressPdfProcessor.cs

@@ -1093,4 +1093,113 @@ public class DevExpressPdfProcessor : IPdfProcessor
     }
 
     #endregion
+
+    #region Add Attachments (Phase 2)
+
+    /// <summary>
+    /// Embeds one or more files as attachments in a PDF document (supports PDF/A-3).
+    /// </summary>
+    public async Task<byte[]> AddAttachmentsAsync(
+        Stream pdfStream,
+        IReadOnlyList<(string FileName, byte[] Content, string? MimeType)> attachments)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+        ArgumentNullException.ThrowIfNull(attachments, nameof(attachments));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        if (attachments.Count == 0)
+            throw new BadRequestException("At least one attachment is required");
+
+        // TODO: Implement AddFileAttachment using DevExpress.Pdf low-level API
+        // Current limitation: DevExpress.Pdf.PdfDocumentProcessor doesn't directly support adding attachments
+        // Workaround options:
+        // 1. Use PdfDocumentProcessor.Document to manipulate PDF structure directly (advanced)
+        // 2. Use third-party library for this specific operation
+        // 3. Wait for DevExpress API update
+
+        throw new NotImplementedException(
+            "Add attachments feature is not yet implemented. " +
+            "DevExpress.Pdf high-level API doesn't directly support adding file attachments. " +
+            "This requires low-level PDF structure manipulation.");
+    }
+
+    private string InferMimeType(string fileName)
+    {
+        string extension = Path.GetExtension(fileName).ToLowerInvariant();
+        return extension switch
+        {
+            ".xml" => "application/xml",
+            ".pdf" => "application/pdf",
+            ".json" => "application/json",
+            ".txt" => "text/plain",
+            ".jpg" or ".jpeg" => "image/jpeg",
+            ".png" => "image/png",
+            _ => "application/octet-stream"
+        };
+    }
+
+    #endregion
+
+    #region PDF Conversion (Phase 3)
+
+    /// <summary>
+    /// Converts a standard PDF to PDF/A format.
+    /// </summary>
+    public async Task<byte[]> ConvertToPdfAAsync(Stream pdfStream, string pdfALevel)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        if (string.IsNullOrWhiteSpace(pdfALevel))
+            throw new BadRequestException("PDF/A level is required");
+
+        // TODO: Implement PDF to PDF/A conversion using DevExpress
+        // Current limitation: DevExpress.Pdf.PdfDocumentProcessor doesn't directly support PDF/A conversion
+        // Requires using specialized PDF/A conversion libraries or low-level PDF manipulation
+
+        throw new NotImplementedException(
+            $"PDF to PDF/A conversion ({pdfALevel}) is not yet implemented. " +
+            "DevExpress.Pdf high-level API doesn't directly support PDF/A conversion. " +
+            "This requires specialized PDF/A conversion logic.");
+    }
+
+    /// <summary>
+    /// Converts a PDF/A document to a standard PDF (removes PDF/A restrictions).
+    /// </summary>
+    public async Task<byte[]> ConvertFromPdfAAsync(Stream pdfStream)
+    {
+        // 1. Validate input
+        ArgumentNullException.ThrowIfNull(pdfStream, nameof(pdfStream));
+
+        if (pdfStream.Length == 0)
+            throw new BadRequestException("PDF stream cannot be empty");
+
+        if (pdfStream.Position != 0)
+            throw new BadRequestException("PDF stream must be at position 0");
+
+        // 2. Load PDF/A
+        using var processor = new PdfDocumentProcessor();
+        processor.LoadDocument(pdfStream);
+
+        // 3. Save as standard PDF
+        // DevExpress SaveDocument without special options creates standard PDF
+        using var outputStream = new MemoryStream();
+        processor.SaveDocument(outputStream);
+        
+        return await Task.FromResult(outputStream.ToArray());
+    }
+
+    #endregion
 }