Add DualInputDocumentFilter for Swagger content merging

Introduced the `DualInputDocumentFilter` class to merge Swagger operations with the same path but different `[Consumes]` attributes (`multipart/form-data` and `application/json`) into a single operation. This ensures both content types are visible in the Swagger UI.

Updated `SwaggerConfiguration.cs` to:
- Resolve conflicting actions by keeping the first variant.
- Register the `DualInputDocumentFilter` to enable content type merging.

DocumentOperator.API/Configuration/SwaggerConfiguration.cs

@@ -24,6 +24,13 @@ namespace DocumentOperator.API.Configuration
                     Description = "PDF Verarbeitungs-Service für Validierung, Stempel, Zertifikate, Anhänge & Zusammenführung"
                 });
 
+                // Resolve conflicting actions: Keep first variant
+                // DualInputDocumentFilter will merge both variants into single operation
+                options.ResolveConflictingActions(apiDescriptions => apiDescriptions.First());
+
+                // Add document filter to merge operations with different content types
+                options.DocumentFilter<DualInputDocumentFilter>();
+
                 // XML-Kommentare einbinden
                 var xmlFile = $"{Assembly.GetExecutingAssembly().GetName().Name}.xml";
                 var xmlPath = Path.Combine(AppContext.BaseDirectory, xmlFile);