Fix type issues in openai_utils.py

[jackgerrits]

Why are these changes needed?

Related issue number

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• 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 76.92308% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 56.11%. Comparing base (e23bdfb) to head (be32d60).

Files	Patch %	Lines
autogen/oai/openai_utils.py	76.92%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2062       +/-   ##
===========================================
+ Coverage   36.87%   56.11%   +19.24%     
===========================================
  Files          68       68               
  Lines        7067     7062        -5     
  Branches     1540     1668      +128     
===========================================
+ Hits         2606     3963     +1357     
+ Misses       4233     2765     -1468     
- Partials      228      334      +106     

Flag	Coverage Δ
unittests	56.03% <76.92%> (+19.15%)	⬆️

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