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.Tests/Unit/Client/PdfAttachmentClientTests.cs

@@ -1,3 +1,4 @@
+using DocumentService.Application.Common.DTOs;
 using DocumentService.Client.Clients;
 using DocumentService.Client.Interfaces;
 using DocumentService.Client.Models.Requests;
@@ -63,7 +64,7 @@ public class PdfAttachmentClientTests
         {
             HasAttachments = true,
             AttachmentCount = 1,
-            Attachments = new List<AttachmentMetadata>
+            Attachments = new List<AttachmentDto>
             {
                 new() { FileName = "factur-x.xml", MimeType = "application/xml", Size = 512 }
             }
@@ -85,7 +86,7 @@ public class PdfAttachmentClientTests
     public async Task CheckAttachmentsAsync_Stream_PdfWithNoAttachments_ReturnsFalse()
     {
         // Arrange
-        var expected = new AttachmentCheckResult { HasAttachments = false, AttachmentCount = 0 };
+        var expected = new AttachmentCheckResult { HasAttachments = false, AttachmentCount = 0, Attachments = [] };
         var (client, _) = BuildJson(expected);
 
         // Act