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/DualInputDocumentFilter.cs

@@ -0,0 +1,125 @@
+using Microsoft.AspNetCore.Mvc.ApiExplorer;
+using Microsoft.OpenApi.Models;
+using Swashbuckle.AspNetCore.SwaggerGen;
+
+namespace DocumentOperator.API.Configuration
+{
+    /// <summary>
+    /// Swagger document filter that merges operations with same path but different [Consumes] attributes.
+    /// Ensures both multipart/form-data and application/json variants are visible in Swagger UI.
+    /// </summary>
+    public class DualInputDocumentFilter : IDocumentFilter
+    {
+        private readonly IApiDescriptionGroupCollectionProvider _apiDescriptionProvider;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DualInputDocumentFilter"/> class.
+        /// </summary>
+        /// <param name="apiDescriptionProvider">API description provider to access all endpoints</param>
+        public DualInputDocumentFilter(IApiDescriptionGroupCollectionProvider apiDescriptionProvider)
+        {
+            _apiDescriptionProvider = apiDescriptionProvider;
+        }
+
+        /// <summary>
+        /// Applies the filter to merge operations with different content types.
+        /// </summary>
+        public void Apply(OpenApiDocument swaggerDoc, DocumentFilterContext context)
+        {
+            var allApiDescriptions = _apiDescriptionProvider.ApiDescriptionGroups.Items
+                .SelectMany(g => g.Items)
+                .ToList();
+
+            // Group by path
+            var groupedByPath = allApiDescriptions
+                .GroupBy(x => "/" + x.RelativePath)
+                .ToList();
+
+            foreach (var group in groupedByPath)
+            {
+                var path = group.Key;
+
+                if (!swaggerDoc.Paths.ContainsKey(path))
+                    continue;
+
+                var pathItem = swaggerDoc.Paths[path];
+
+                // Find multipart and JSON variants
+                var multipartDesc = group.FirstOrDefault(x =>
+                    x.SupportedRequestFormats.Any(f => f.MediaType == "multipart/form-data"));
+
+                var jsonDesc = group.FirstOrDefault(x =>
+                    x.SupportedRequestFormats.Any(f => f.MediaType == "application/json"));
+
+                // If we have both variants, merge them into single operation
+                if (multipartDesc != null && jsonDesc != null)
+                {
+                    var httpMethod = multipartDesc.HttpMethod?.ToLowerInvariant();
+                    OperationType operationType;
+
+                    if (!Enum.TryParse<OperationType>(httpMethod, true, out operationType))
+                        continue;
+
+                    if (!pathItem.Operations.ContainsKey(operationType))
+                        continue;
+
+                    var operation = pathItem.Operations[operationType];
+
+                    // Ensure RequestBody exists
+                    if (operation.RequestBody == null)
+                    {
+                        operation.RequestBody = new OpenApiRequestBody
+                        {
+                            Required = true,
+                            Content = new Dictionary<string, OpenApiMediaType>()
+                        };
+                    }
+
+                    // Add multipart/form-data if missing
+                    if (!operation.RequestBody.Content.ContainsKey("multipart/form-data"))
+                    {
+                        operation.RequestBody.Content.Add("multipart/form-data", new OpenApiMediaType
+                        {
+                            Schema = new OpenApiSchema
+                            {
+                                Type = "object",
+                                Properties = new Dictionary<string, OpenApiSchema>
+                                {
+                                    ["file"] = new OpenApiSchema
+                                    {
+                                        Type = "string",
+                                        Format = "binary",
+                                        Description = "PDF file to upload"
+                                    }
+                                },
+                                Required = new HashSet<string> { "file" }
+                            }
+                        });
+                    }
+
+                    // Add application/json if missing
+                    if (!operation.RequestBody.Content.ContainsKey("application/json"))
+                    {
+                        operation.RequestBody.Content.Add("application/json", new OpenApiMediaType
+                        {
+                            Schema = new OpenApiSchema
+                            {
+                                Type = "object",
+                                Properties = new Dictionary<string, OpenApiSchema>
+                                {
+                                    ["base64Pdf"] = new OpenApiSchema
+                                    {
+                                        Type = "string",
+                                        Format = "byte",
+                                        Description = "Base64-encoded PDF file content"
+                                    }
+                                },
+                                Required = new HashSet<string> { "base64Pdf" }
+                            }
+                        });
+                    }
+                }
+            }
+        }
+    }
+}

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);