feat(api): add GPT-6.1 Sol model identifier (#3986)

## Summary

Adds gpt-6.1-sol to the known model identifiers for Chat Completions and
Responses.

## Changes

- Add gpt-6.1-sol to the shared ChatModel type.

Co-authored-by: markstuart-oai <323302876+markstuart-oai@users.noreply.github.com>