[Core] Extending inline docs for openai_utils.py, boosting coverage in test_utils.py, fixing #762

[maxim-saplin]

Why are these changes needed?

• Improved docstrings for all config_list creation functions declared in openai_utils.py
• test_utils.py has more cases tested ramping up coverage
• test_utils.py no longer depends on files that are part of .gitignore (and hence not downloaded when cloning), utilises temp files and fake data
• openai_utils.get_config_list() has an assertion verifying that number of items in files is equal (replacing the current behaviour with out of index being thrown)

Related issue number

Closes #762

Checks

• (NOT RELEVANT) 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.
  P.S. settings up the full suite with all required open configs is a challenge, though I made sure the parts and tests impacted by the changes ran fine

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9388bae) 30.14% compared to head (857e955) 40.96%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1046       +/-   ##
===========================================
+ Coverage   30.14%   40.96%   +10.81%     
===========================================
  Files          30       30               
  Lines        4020     4028        +8     
  Branches      908      960       +52     
===========================================
+ Hits         1212     1650      +438     
+ Misses       2729     2252      -477     
- Partials       79      126       +47     

Flag	Coverage Δ
unittests	40.91% <100.00%> (+10.81%)	⬆️

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[ekzhu]

Ok