Fix code review: use null format (not empty string) and fix XML doc href

Author: Copilot

src/Core/Services/OpenAPI/OpenApiDocumentor.cs

@@ -1486,13 +1486,13 @@ private static OpenApiSchema CreateComponentSchema(
                 if (metadataProvider.TryGetBackingColumn(entityName, field, out string? backingColumnValue) && !string.IsNullOrEmpty(backingColumnValue))
                 {
                     string typeMetadata = string.Empty;
-                    string formatMetadata = string.Empty;
+                    string? formatMetadata = null;
                     string? fieldDescription = null;
 
                     if (dbObject.SourceDefinition.Columns.TryGetValue(backingColumnValue, out ColumnDefinition? columnDef))
                     {
                         typeMetadata = TypeHelper.GetJsonDataTypeFromSystemType(columnDef.SystemType).ToString().ToLower();
-                        formatMetadata = TypeHelper.GetOpenApiFormatFromSystemType(columnDef.SystemType, columnDef.DbType) ?? string.Empty;
+                        formatMetadata = TypeHelper.GetOpenApiFormatFromSystemType(columnDef.SystemType, columnDef.DbType);
                     }
 
                     if (entityConfig?.Fields != null)

src/Core/Services/TypeHelper.cs

@@ -212,7 +212,7 @@ public static EdmPrimitiveTypeKind GetEdmPrimitiveTypeFromITypeNode(ITypeNode co
         /// </summary>
         /// <param name="type">CLR type</param>
         /// <param name="dbType">Optional DbType used to distinguish date-only (DbType.Date) from date-time types.</param>
-        /// <seealso cref="https://spec.openapis.org/oas/v3.0.1#data-types"/>
+        /// <seealso href="https://spec.openapis.org/oas/v3.0.1#data-types"/>
         /// <returns>OpenAPI format string (e.g. "date-time", "date", "time", "uuid", "byte"),
         /// or null if no standard format applies to the type.</returns>
         public static string? GetOpenApiFormatFromSystemType(Type type, DbType? dbType = null)