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/Common/Interfaces/IPdfProcessor.cs

@@ -13,4 +13,14 @@ public interface IPdfProcessor
     /// Thrown when PDF is corrupted or cannot be processed
     /// </exception>
     Task<PdfMetadata> ValidateAsync(byte[] pdfBytes);
+
+    /// <summary>
+    /// Validates a PDF/A document and checks conformance level.
+    /// </summary>
+    /// <param name="pdfBytes">PDF content as byte array</param>
+    /// <returns>PDF/A metadata including conformance level and validation errors/warnings</returns>
+    /// <exception cref="Domain.Common.Exceptions.PdfProcessingException">
+    /// Thrown when PDF is corrupted or cannot be processed
+    /// </exception>
+    Task<PdfAMetadata> ValidatePdfAAsync(byte[] pdfBytes);
 }