[Core] Extending inline docs for `openai_utils.py`, boosting coverage in `test_utils.py`, fixing #762 (#1046)

* fix for #762 ,get_config_list/oia assetion, docs, test

* Improved docstrings

* test_utils using fake data and temp files

* "Black" formatting applied

* Fix build (KEY_LOC and OAI_CONFIG_LIST as consts in test_client)

* Ramping up openai_utils coverage

* Missing parameter doc.

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Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>