feat(openai): add gpt-5.5 to model registry

[xtreme-sameer-vohra]

Summary

Adds gpt-5.5 to the OpenAI model registry so it can be selected via the livekit-plugins-openai plugin (openai.LLM(model="gpt-5.5")).

The model is also added to _supports_reasoning_effort(), matching the other bare gpt-5.x entries so reasoning_effort is passed through.

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