Migrate to DevExpress.Document.Processor library

Replaced `DevExpress.Pdf.Core` with `DevExpress.Document.Processor` in `DocumentOperator.Infrastructure.csproj` to adopt a newer library for PDF processing. Removed the `Services\PdfProcessing\` folder reference.

Updated `DevExpressPdfProcessorTests` to reflect changes in the `ValidateAsync` method's behavior.

Introduced the `DevExpressPdfProcessor` class, implementing the `IPdfProcessor` interface. This class validates PDF documents and extracts metadata using the `DevExpress.Pdf` library. Added defensive input validation, metadata extraction, and exception handling for domain consistency.

DocumentOperator.Infrastructure/Services/PdfProcessing/DevExpressPdfProcessor.cs

@@ -0,0 +1,67 @@
+using DevExpress.Pdf;
+using DevExpress.Pdf;
+using DocumentOperator.Application.Common.Interfaces;
+using DocumentOperator.Domain.Common.Exceptions;
+
+namespace DocumentOperator.Infrastructure.Services.PdfProcessing;
+
+/// <summary>
+/// PDF processor implementation using DevExpress.Pdf library.
+/// Handles PDF validation and metadata extraction.
+/// </summary>
+public class DevExpressPdfProcessor : IPdfProcessor
+{
+    /// <summary>
+    /// Validates a PDF document and returns metadata.
+    /// </summary>
+    /// <param name="pdfBytes">PDF content as byte array</param>
+    /// <returns>PDF metadata (page count, file size, version, etc.)</returns>
+    /// <exception cref="PdfProcessingException">Thrown when PDF is invalid or null</exception>
+    public async Task<DocumentOperator.Domain.Models.ValueObjects.PdfMetadata> ValidateAsync(byte[] pdfBytes)
+    {
+        // 1. Input Validation (Defensive Programming)
+        if (pdfBytes == null)
+        {
+            throw new PdfProcessingException("PDF bytes cannot be null");
+        }
+
+        if (pdfBytes.Length == 0)
+        {
+            throw new PdfProcessingException("PDF bytes cannot be empty");
+        }
+
+        try
+        {
+            // 2. Load PDF with DevExpress Document API (PdfDocumentProcessor)
+            using var processor = new PdfDocumentProcessor();
+            processor.LoadDocument(new MemoryStream(pdfBytes));
+
+            // 3. Extract metadata
+            var document = processor.Document;
+
+            int pageCount = document.Pages.Count;
+            string pdfVersion = document.Version.ToString(); // z.B. "1.4", "1.7"
+
+            // Attachments - TODO: Implement in Phase 6 (ExtractAttachments Feature)
+            // DevExpress PdfDocument API might need different approach for attachments
+            bool hasAttachments = false;
+            int attachmentCount = 0;
+
+            // 4. Create and return PdfMetadata Value Object (fully qualified name!)
+            return new DocumentOperator.Domain.Models.ValueObjects.PdfMetadata(
+                pageCount: pageCount,
+                fileSizeBytes: pdfBytes.Length,
+                pdfVersion: pdfVersion,
+                hasAttachments: hasAttachments,
+                attachmentCount: attachmentCount
+            );
+        }
+        catch (Exception ex) when (ex is not PdfProcessingException)
+        {
+            // Wrap DevExpress exceptions in our domain exception
+            throw new PdfProcessingException(
+                $"Failed to validate PDF: {ex.Message}",
+                ex);
+        }
+    }
+}