Fix isFreeFormObject check for allOf, oneOf, anyOf (#2044)

* fix free form object detection

* refator

Author: wing328

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -534,6 +534,15 @@ public static boolean isFreeFormObject(Schema schema) {
             return false;
         }
 
+        // not free-form if allOf, anyOf, oneOf is not empty
+        if (schema instanceof ComposedSchema) {
+            ComposedSchema cs = (ComposedSchema) schema;
+            List<Schema> interfaces = getInterfaces(cs);
+            if (interfaces != null && !interfaces.isEmpty()) {
+                return false;
+            }
+        }
+
         // has at least one property
         if ("object".equals(schema.getType())) {
             // no properties