fix: Normalize openai client stop reason to make more robust

[jackgerrits]

Closes #5020
Closes #4875

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.49%. Comparing base (a7c97db) to head (4042989).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...xt/src/autogen_ext/models/openai/_openai_client.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5027      +/-   ##
==========================================
- Coverage   72.22%   68.49%   -3.73%     
==========================================
  Files         112      156      +44     
  Lines        6563    10129    +3566     
==========================================
+ Hits         4740     6938    +2198     
- Misses       1823     3191    +1368     

Flag	Coverage Δ
unittests	68.49% <80.00%> (-3.73%)	⬇️

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