[codex] Add B.AI LLM connection support

[aquadrop]

Summary

• Add a bai/ model prefix for routing requests to B.AI's OpenAI-compatible LLM API.
• Map routed B.AI models to LiteLLM's openai/ provider with B.AI API defaults from BAI_API_KEY and optional BAI_API_BASE.
• Document B.AI setup and add mocked unit coverage for sync, async, explicit overrides, and non-B.AI behavior.

Validation

• python -m pytest routellm/tests/test_bai_support.py