### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

When using LiteLLM model group fallbacks from a Gemini model to an OpenAI model with the OpenAI Agents SDK, fallback fails if the agent performs tool calls. ``` litellm.BadRequestError: OpenAIException - Invalid schema for function \'my_tool\': In context=(), \'additionalProperties\' is required to be supplied and to be false. ``` * Using OpenAI and Gemini models as the initial model work as expected. * Fallbacks from OpenAI models to Gemini models work as expected.

### Steps to Reproduce

``` from __future__ import annotations

import os import shutil import socket import subprocess import sys import tempfile import time import urllib.error import urllib.request from contextlib import contextmanager from pathlib import Path from typing import Annotated, Iterator from unittest import SkipTest

from agents import Agent, ModelSettings, RunConfig, Runner, function_tool from agents.extensions.models.litellm_model import LitellmModel from dotenv import load_dotenv from pydantic import Field

GATEWAY_KEY = "sk-litellm-test" GATEWAY_KEY_ENV = "LITE…