feat(api): add GPT-6 Sol and Luna model identifiers (#3935)

## Summary

Adds gpt-6-sol and gpt-6-luna to the known Chat Completions and
Responses model identifiers.

## Changes

- Add the two identifiers to Python model type hints.
- Continue accepting existing model identifiers and arbitrary model
strings.

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