feat: Add PDF/A validation endpoint (Feature 3)

Domain layer:

  - PdfAMetadata value object (isValid, pdfVersion, pageCount, encrypted, pdfaVersion, pdfaCompliant, errors, warnings)

Infrastructure layer:

  - IPdfProcessor.ValidatePdfAAsync() interface method

  - DevExpressPdfProcessor.ValidatePdfAAsync() implementation

  - DetectEncryption() - scans PDF raw data for /Encrypt keyword

  - DetectPdfAConformance() - parses XMP metadata (pdfaid:part, pdfaid:conformance)

  - Validation: encrypted PDF cannot be PDF/A compliant

Application layer:

  - ValidatePdfAQuery + ValidatePdfAQueryHandler (co-located)

  - ValidatePdfAQueryValidator (FluentValidation: PdfBytes XOR Base64Pdf)

  - PdfAValidationResult DTO

  - AutoMapper: PdfAMetadata -> PdfAValidationResult

API layer:

  - PdfValidationController.ValidatePdfAFromFile() (multipart/form-data)

  - PdfValidationController.ValidatePdfAFromBase64() (application/json)

  - XML documentation with response codes

Result:

  - POST /api/pdf/validation/validate-pdfa (both multipart and JSON)

  - Returns: conformance level, errors, warnings

  - Build: 0 errors, 4 warnings (DevExpress eval)

  - Tests: 20/20 passing

Next: Integration tests + Swagger test case

DocumentOperator.Application/ValidatePdfA/Queries/ValidatePdfAQuery.cs

@@ -0,0 +1,45 @@
+using AutoMapper;
+using DocumentOperator.Application.Common.DTOs;
+using DocumentOperator.Application.Common.Interfaces;
+using MediatR;
+
+namespace DocumentOperator.Application.ValidatePdfA.Queries;
+
+/// <summary>
+/// Query for PDF/A validation (supports both byte array and Base64 input)
+/// </summary>
+public record ValidatePdfAQuery : IRequest<PdfAValidationResult>
+{
+    /// <summary>
+    /// PDF as byte array (direct upload)
+    /// </summary>
+    public byte[]? PdfBytes { get; init; }
+
+    /// <summary>
+    /// PDF as Base64 string (API clients)
+    /// </summary>
+    public string? Base64Pdf { get; init; }
+}
+
+/// <summary>
+/// Handler for ValidatePdfAQuery
+/// Orchestrates PDF/A validation using IPdfProcessor and AutoMapper
+/// </summary>
+public class ValidatePdfAQueryHandler(IPdfProcessor PdfProcessor, IMapper Mapper) 
+    : IRequestHandler<ValidatePdfAQuery, PdfAValidationResult>
+{
+    /// <summary>
+    /// Validates PDF/A and returns metadata with conformance level
+    /// </summary>
+    public async Task<PdfAValidationResult> Handle(ValidatePdfAQuery request, CancellationToken cancellationToken)
+    {
+        // Use byte[] if available, otherwise convert Base64
+        byte[] pdfBytes = request.PdfBytes ?? Convert.FromBase64String(request.Base64Pdf!);
+
+        // Call DevExpress service (can throw PdfProcessingException)
+        var metadata = await PdfProcessor.ValidatePdfAAsync(pdfBytes);
+
+        // Map domain entity to DTO using AutoMapper
+        return Mapper.Map<PdfAValidationResult>(metadata);
+    }
+}