Fix tests for GPT assistant

[davorrunje]

Why are these changes needed?

Fixes failing tests for GPT assistant.

Steps to reproduce

Run tests on test/agentchat/contrib/test_gpt_assistant.py

git checkout main
git pull
pytest test/agentchat/contrib/test_gpt_assistant.py

Output:

======================================= test session starts ========================================
platform linux -- Python 3.10.13, pytest-7.4.4, pluggy-1.4.0
rootdir: /workspaces/autogen
configfile: pyproject.toml
plugins: asyncio-0.23.3, anyio-4.2.0
asyncio: mode=strict
collected 7 items                                                                                  

test/agentchat/contrib/test_gpt_assistant.py .....FF                                         [100%]

Related issue number

Closes #1504

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[davorrunje]

@sonichi can you please restart the tests? Everything is working locally.