update description text

openapi.yaml

@@ -10003,6 +10003,7 @@ components:
         Controls which (if any) tool is called by the model.
         `none` means the model will not call any tools and instead generates a message.
         `auto` is the default value and means the model can pick between generating a message or calling a tool.
+        `required` means the model must pick one of the available tools or functions before responding to the user.
         Specifying a particular tool like `{"type": "file_search"}` or `{"type": "function", "function": {"name": "my_function"}}` forces the model to call that tool.
 
       oneOf: