Add ValidatePDF feature with API, Swagger, and tests

Implemented the ValidatePDF feature end-to-end:
- Added `/api/v1/documents/validate` Minimal API endpoint.
- Introduced centralized ExceptionHandlingMiddleware.
- Configured Swagger with `AddSwaggerDocumentation` extension.
- Enabled XML comments in `DocumentOperator.API.csproj`.
- Updated DTOs with XML comments and added `FileSizeMB`.
- Added integration tests for the ValidatePDF endpoint (3 tests).
- Registered infrastructure services (e.g., `IPdfProcessor`).
- Refactored `Program.cs` to include middleware and endpoints.
- Updated PHASENPLAN.md and ROADMAP.md to reflect progress.
- Cleaned up code and made `Program` accessible for tests.

DocumentOperator.Application/Common/DTOs/ValidatePdfRequest.cs

@@ -1,7 +1,7 @@
 namespace DocumentOperator.Application.Common.DTOs;
 
 /// <summary>
-/// Request DTO for ValidatePdf endpoint
+/// Request für PDF-Validierung
 /// </summary>
-/// <param name="Base64Pdf">PDF content as Base64 string</param>
+/// <param name="Base64Pdf">Base64-encodiertes PDF-Dokument</param>
 public record ValidatePdfRequest(string Base64Pdf);

DocumentOperator.Application/Common/DTOs/ValidatePdfResponse.cs

@@ -1,14 +1,18 @@
 namespace DocumentOperator.Application.Common.DTOs;
 
 /// <summary>
-/// Response DTO for ValidatePdf endpoint
-/// Contains PDF metadata
+/// 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>
 public record ValidatePdfResponse(
     int PageCount,
     long FileSizeBytes,
     double FileSizeMB,
     string PdfVersion,
     bool HasAttachments,
-    int AttachmentCount
-);
+    int AttachmentCount);