* Support legacy OpenAI base URL env var
* Add custom OpenAI-compatible endpoint support
* Refactor OpenAI completion module test to restore original module state
- Added logic to save and restore the original OpenAI completion module during the test to prevent issues with class re-imports affecting subsequent tests.
- Ensured that the test checks for the presence of the module and its attributes only after the module is properly reloaded.
- Improved test reliability by avoiding potential failures due to module state changes across tests.
* addressing comments
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