Refactor DTOs and tests; add framework-specific handling

Refactored the codebase to use shared DTOs from `DocumentService.Application.Common.DTOs`, removing inline DTO definitions in client interfaces and tests. Introduced conditional compilation to handle framework-specific differences in DTOs and handlers.

Updated test cases to align with the new DTO structures, including `SwissQrCodeExtractionResult`, `PdfValidationResult`, and `ZugferdExtractionResult`. Adjusted test data to match updated DTO properties.

Cleaned up solution and project files, ensuring proper references and organization. Removed unused files and fixed project references in `DocumentService.Tests.csproj`. Improved maintainability and consistency across the codebase.

DocumentService.Client/Interfaces/IPdfAttachmentClient.cs

@@ -1,3 +1,4 @@
+using DocumentService.Application.Common.DTOs;
 using DocumentService.Client.Models.Requests;
 
 namespace DocumentService.Client.Interfaces;
@@ -60,30 +61,4 @@ public interface IPdfAttachmentClient
     /// <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; }
-}
+}