Add interfaces and DTOs for PDF operations support

Introduced multiple interfaces and DTOs to support a wide range of
PDF-related operations, including attachment handling, conversion,
validation, annotation, stamping, and metadata extraction.

Key changes:
- Added `DocumentServiceClientOptions` for HTTP client config.
- Introduced `IPdfAttachmentClient` for attachment operations.
- Added `IPdfConversionClient` for PDF/A conversion (marked obsolete).
- Added `IPdfOperationsClient` for merge, annotate, and stamp ops.
- Introduced `IPdfValidationClient` for PDF and PDF/A validation.
- Added `ISwissQrCodeClient` for Swiss QR Code extraction.
- Added `IZugferdClient` for ZUGFeRD detection and extraction.
- Created request DTOs for Base64-encoded operations.
- Added enums for annotation, stamp, and validation configurations.

These changes provide a flexible and extensible foundation for
interacting with PDF documents, supporting both multipart and
Base64-encoded inputs, and ensuring type safety with enums and records.

DocumentService.Client/Interfaces/IPdfValidationClient.cs

@@ -0,0 +1,75 @@
+namespace DocumentService.Client.Interfaces;
+
+/// <summary>
+/// Client for PDF validation operations (validate, validate PDF/A).
+/// </summary>
+public interface IPdfValidationClient
+{
+    /// <summary>
+    /// Validates a PDF from stream (multipart upload).
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Validation result with metadata</returns>
+    Task<PdfValidationResult> ValidatePdfAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Validates a PDF from byte array (Base64 JSON).
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Validation result with metadata</returns>
+    Task<PdfValidationResult> ValidatePdfAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Validates a PDF/A from stream (multipart upload).
+    /// </summary>
+    /// <param name="pdfStream">PDF/A file stream</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A validation result with conformance level and errors</returns>
+    Task<PdfAValidationResult> ValidatePdfAAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Validates a PDF/A from byte array (Base64 JSON).
+    /// </summary>
+    /// <param name="pdfBytes">PDF/A file as byte array</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>PDF/A validation result with conformance level and errors</returns>
+    Task<PdfAValidationResult> ValidatePdfAAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// DTO for PDF validation result
+/// </summary>
+public record PdfValidationResult
+{
+    /// <summary>Total number of pages in the document.</summary>
+    public int PageCount { get; init; }
+    /// <summary>File size in bytes.</summary>
+    public long FileSizeBytes { get; init; }
+    /// <summary>PDF specification version (e.g. "1.7").</summary>
+    public string PdfVersion { get; init; } = string.Empty;
+    /// <summary>Whether the document is password-protected.</summary>
+    public bool IsEncrypted { get; init; }
+    /// <summary>Whether the document contains embedded file attachments.</summary>
+    public bool HasAttachments { get; init; }
+    /// <summary>Number of embedded file attachments.</summary>
+    public int AttachmentCount { get; init; }
+}
+
+/// <summary>
+/// DTO for PDF/A validation result
+/// </summary>
+public record PdfAValidationResult
+{
+    /// <summary>Whether the document is fully PDF/A conformant.</summary>
+    public bool IsValid { get; init; }
+    /// <summary>Detected PDF/A conformance level (e.g. "PDF/A-3b"), or <c>null</c> if not a PDF/A document.</summary>
+    public string? PdfAVersion { get; init; }
+    /// <summary>Total number of pages in the document.</summary>
+    public int PageCount { get; init; }
+    /// <summary>List of conformance errors found during validation.</summary>
+    public List<string> Errors { get; init; } = new();
+    /// <summary>List of conformance warnings found during validation.</summary>
+    public List<string> Warnings { get; init; } = new();
+}