Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Application/Common/DTOs/PdfValidationResult.cs

@@ -0,0 +1,20 @@
+namespace DocumentService.Application.Common.DTOs;
+
+/// <summary>
+/// Response mit PDF-Metadaten
+/// </summary>
+/// <param name="PageCount">Anzahl der Seiten</param>
+/// <param name="FileSizeBytes">Dateigröße in Bytes</param>
+/// <param name="FileSizeMB">Dateigröße in MB (gerundet auf 2 Dezimalstellen)</param>
+/// <param name="PdfVersion">PDF-Version (z.B. "1.4")</param>
+/// <param name="HasAttachments">Hat das PDF Anhänge?</param>
+/// <param name="AttachmentCount">Anzahl der Anhänge</param>
+/// <param name="IsEncrypted">Ist das PDF passwortgeschützt/verschlüsselt?</param>
+public record PdfValidationResult(
+    int PageCount,
+    long FileSizeBytes,
+    double FileSizeMB,
+    string PdfVersion,
+    bool HasAttachments,
+    int AttachmentCount,
+    bool IsEncrypted);