Fix the assistant test case error caused by openai incompatible change

[IANTHEREAL]

Why are these changes needed?

OpenAI Assistant Service make some incompatible changes, it would cause failing to create assistant, if the function name is not unique. One case scene: https://github.com/microsoft/autogen/actions/runs/9074961126/job/24934934702?pr=2672

Because there was a problem with my openai account, I did not conduct a comprehensive test, but I passed the two failed cases. Hope this pr can fix the case failure.

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.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 25.29%. Comparing base (19de99e) to head (6cb2207).

Files	Patch %	Lines
autogen/agentchat/contrib/gpt_assistant_agent.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2718      +/-   ##
==========================================
- Coverage   34.36%   25.29%   -9.07%     
==========================================
  Files          87       93       +6     
  Lines        9336     9420      +84     
  Branches     1987     2142     +155     
==========================================
- Hits         3208     2383     -825     
- Misses       5862     6763     +901     
- Partials      266      274       +8     

Flag	Coverage Δ
unittest	12.70% <0.00%> (?)
unittests	24.52% <0.00%> (-9.84%)	⬇️

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