Reapply "add warn logs when x-kotlin-implements-fields is used without x-kotlin-implements to improve usability"

This reverts commit 50bb7e5.

Author: Picazsoo

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractKotlinCodegen.java

@@ -836,9 +836,15 @@ protected boolean needToImport(String type) {
     public CodegenModel fromModel(String name, Schema schema) {
         CodegenModel m = super.fromModel(name, schema);
         List<String> implementedInterfacesClasses = (List<String>) m.getVendorExtensions().getOrDefault(VendorExtension.X_KOTLIN_IMPLEMENTS.getName(), List.of());
-        List<String> implementedInterfacesFields = implementedInterfacesClasses.isEmpty()
-                ? List.of()
-                : (List<String>) m.getVendorExtensions().getOrDefault(VendorExtension.X_KOTLIN_IMPLEMENTS_FIELDS.getName(), List.of());
+        List<String> implementedInterfacesFields = Optional.ofNullable((List<String>) m.getVendorExtensions().get(VendorExtension.X_KOTLIN_IMPLEMENTS_FIELDS.getName()))
+                .map(xKotlinImplementsFields -> {
+                    if (implementedInterfacesClasses.isEmpty() && !xKotlinImplementsFields.isEmpty()) {
+                        LOGGER.warn("Annotating {} with {} without {} is not supported. {} will be ignored.",
+                                name, VendorExtension.X_KOTLIN_IMPLEMENTS_FIELDS.getName(), VendorExtension.X_KOTLIN_IMPLEMENTS.getName(),
+                                VendorExtension.X_KOTLIN_IMPLEMENTS_FIELDS.getName());
+                    }
+                    return xKotlinImplementsFields;
+                }).orElse(List.of());
         m.optionalVars = m.optionalVars.stream().distinct().collect(Collectors.toList());
         // Update allVars/requiredVars/optionalVars with isInherited
         // Each of these lists contains elements that are similar, but they are all cloned