Overwrite API type on model level (GPT codex)

[zikajk]

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Some models (e.g.gpt-5.2-codex in Github Copilot) use different API than the other models.
Right now we use openai-chat with Copilot and that makes Codex impossible to use.

[ericdallo]

Hum, yes, we may wanna have a flag for models to override that