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/Validators/ValidatePdfAQueryValidator.cs

@@ -0,0 +1,18 @@
+using FluentValidation;
+
+namespace DocumentOperator.Application.ValidatePdfA.Validators;
+
+/// <summary>
+/// Validator for ValidatePdfAQuery
+/// Ensures exactly ONE input format is provided (PdfBytes XOR Base64Pdf)
+/// </summary>
+public class ValidatePdfAQueryValidator : AbstractValidator<Queries.ValidatePdfAQuery>
+{
+    public ValidatePdfAQueryValidator()
+    {
+        RuleFor(x => x)
+            .Must(x => (x.PdfBytes != null && x.PdfBytes.Length > 0) ^ 
+                       !string.IsNullOrWhiteSpace(x.Base64Pdf))
+            .WithMessage("Either PdfBytes or Base64Pdf must be provided, but not both");
+    }
+}