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.Domain/Models/ValueObjects/PdfAMetadata.cs

@@ -0,0 +1,47 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+/// <summary>
+/// PDF/A validation metadata including conformance level and validation errors/warnings
+/// </summary>
+public sealed class PdfAMetadata
+{
+    public bool IsValid { get; }
+    public string PdfVersion { get; }
+    public int PageCount { get; }
+    public long FileSizeBytes { get; }
+    public bool Encrypted { get; }
+    public string? PdfAVersion { get; }
+    public bool PdfACompliant { get; }
+    public IReadOnlyList<string> Errors { get; }
+    public IReadOnlyList<string> Warnings { get; }
+
+    // Computed property
+    public double FileSizeMB => FileSizeBytes / 1024.0 / 1024.0;
+
+    public PdfAMetadata(
+        bool isValid,
+        string pdfVersion,
+        int pageCount,
+        long fileSizeBytes,
+        bool encrypted,
+        string? pdfaVersion,
+        bool pdfaCompliant,
+        IReadOnlyList<string> errors,
+        IReadOnlyList<string> warnings)
+    {
+        IsValid = isValid;
+        PdfVersion = pdfVersion;
+        PageCount = pageCount;
+        FileSizeBytes = fileSizeBytes;
+        Encrypted = encrypted;
+        PdfAVersion = pdfaVersion;
+        PdfACompliant = pdfaCompliant;
+        Errors = errors;
+        Warnings = warnings;
+    }
+
+    public override string ToString()
+    {
+        return $"PDF/A: {PdfAVersion ?? "None"}, {PageCount} pages, {FileSizeMB:F2} MB, Compliant: {PdfACompliant}";
+    }
+}