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/IPdfAttachmentClient.cs

@@ -0,0 +1,89 @@
+using DocumentService.Client.Models.Requests;
+
+namespace DocumentService.Client.Interfaces;
+
+/// <summary>
+/// Client for PDF attachment operations (check, extract, add).
+/// </summary>
+public interface IPdfAttachmentClient
+{
+    /// <summary>
+    /// Checks if PDF contains attachments (multipart).
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Attachment check result with metadata</returns>
+    Task<AttachmentCheckResult> CheckAttachmentsAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Checks if PDF contains attachments (Base64).
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Attachment check result with metadata</returns>
+    Task<AttachmentCheckResult> CheckAttachmentsAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extracts all embedded attachments from a PDF (multipart).
+    /// The returned dictionary maps each file name to its decompressed content stream.
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Dictionary of file name ? content stream for each extracted attachment</returns>
+    Task<Dictionary<string, Stream>> ExtractAttachmentsAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Extracts all embedded attachments from a PDF (Base64).
+    /// The returned dictionary maps each file name to its decompressed content stream.
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Dictionary of file name ? content stream for each extracted attachment</returns>
+    Task<Dictionary<string, Stream>> ExtractAttachmentsAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds attachments to PDF (multipart). ?? Not implemented yet in API.
+    /// </summary>
+    /// <param name="pdfStream">PDF file stream</param>
+    /// <param name="attachments">Attachments to add</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Modified PDF as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> AddAttachmentsAsync(Stream pdfStream, List<AttachmentRequestDto> attachments, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Adds attachments to PDF (Base64). ?? Not implemented yet in API.
+    /// </summary>
+    /// <param name="pdfBytes">PDF file as byte array</param>
+    /// <param name="attachments">Attachments to add</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Modified PDF as stream</returns>
+    [Obsolete("API endpoint not implemented yet")]
+    Task<Stream> AddAttachmentsAsync(byte[] pdfBytes, List<AttachmentRequestDto> attachments, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// DTO for attachment check result
+/// </summary>
+public record AttachmentCheckResult
+{
+    /// <summary>Whether the document contains any embedded file attachments.</summary>
+    public bool HasAttachments { get; init; }
+    /// <summary>Number of embedded file attachments found.</summary>
+    public int AttachmentCount { get; init; }
+    /// <summary>Metadata for each attachment found in the document.</summary>
+    public List<AttachmentMetadata> Attachments { get; init; } = new();
+}
+
+/// <summary>
+/// DTO for attachment metadata
+/// </summary>
+public record AttachmentMetadata
+{
+    /// <summary>Name of the embedded file.</summary>
+    public string FileName { get; init; } = string.Empty;
+    /// <summary>MIME type of the embedded file (e.g. "application/xml"), or <c>null</c> if unknown.</summary>
+    public string? MimeType { get; init; }
+    /// <summary>Size of the embedded file in bytes.</summary>
+    public long Size { get; init; }
+}