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.API/Configuration/SwaggerConfiguration.cs

@@ -1,6 +1,28 @@
-namespace DocumentOperator.API.Configuration
+using Microsoft.OpenApi.Models;
+using System.Reflection;
+
+namespace DocumentOperator.API.Configuration
 {
-    public class SwaggerConfiguration
+    public static class SwaggerConfiguration
     {
+        public static IServiceCollection AddSwaggerDocumentation(this IServiceCollection services)
+        {
+            services.AddSwaggerGen(options =>
+            {
+                options.SwaggerDoc("v1", new OpenApiInfo
+                {
+                    Title = "DD Document Operator API",
+                    Version = "v1",
+                    Description = "PDF Verarbeitungs-Service für Validierung, Stempel, Zertifikate, Anhänge & Zusammenführung"
+                });
+
+                // XML-Kommentare einbinden
+                var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
+                var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);
+                options.IncludeXmlComments(xmlPath);
+            });
+
+            return services;
+        }
     }
 }