fix: Structured output with tool calls for OpenAIChatCompletionClient

[ekzhu]

Resolves: #5568

Also, refactored some unit tests.

Integration tests against OpenAI endpoint passed: https://github.com/microsoft/autogen/actions/runs/13484492096

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

[codecov]

Codecov Report

Attention: Patch coverage is 85.54217% with 12 lines in your changes missing coverage. Please review.

Project coverage is 75.58%. Comparing base (a226966) to head (10a0a71).

Files with missing lines	Patch %	Lines
...xt/src/autogen_ext/models/openai/_openai_client.py	85.54%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5671      +/-   ##
==========================================
+ Coverage   75.56%   75.58%   +0.02%     
==========================================
  Files         171      171              
  Lines       10483    10511      +28     
==========================================
+ Hits         7921     7945      +24     
- Misses       2562     2566       +4     

Flag	Coverage Δ
unittests	75.58% <85.54%> (+0.02%)	⬆️

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