Bump LiteLLM to fix Vertex AI function calling with no-param tools

[habema]

Fixes a Vertex AI error that occurs within LiteLLM, where tools with no parameters (like get_random_number()) would fail with a BadRequestError about function declaration parameters schema needing to be type OBJECT.

Bumped LiteLLM dependency to 1.81.0 which handles this case properly. Related bug fix here.

Error:

litellm.BadRequestError: Vertex_aiException BadRequestError - {
  "error": {
    "code": 400,
    "message": "Unable to submit request because `get_random_number` functionDeclaration parameters schema should be of type OBJECT. Learn more: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/function-calling",
    "status": "INVALID_ARGUMENT"
  }
}

Repro Script:

import asyncio
import random

from agents import Agent, Runner, function_tool


@function_tool
def get_random_number() -> int:
    """
    Returns a random number between 0 and 100.
    """
    return random.randint(0, 100)


agent = Agent(
    name="Assistant",
    model="litellm/vertex_ai/gemini-3-flash-preview",
    instructions="You are a helpful agent.",
    tools=[get_random_number],
)


async def main():
    result = await Runner.run(agent, input="Call the `get_random_number` tool")
    print(result.final_output)


if __name__ == "__main__":
    asyncio.run(main())