chore(api): document supported image generation models (#3695)

## Summary

Refresh the generated API reference and document the supported
image-generation model names in the stable and beta Responses tool
types. The SDK already accepts these model names; this changes
documentation, not public type signatures or runtime behavior.

## Changes

- Document `gpt-image-1`, `gpt-image-1-mini`, `gpt-image-1.5`,
`gpt-image-2`, `gpt-image-2-2026-04-21`, and `chatgpt-image-latest`; the
documented default remains `gpt-image-1`.
- Refresh the transformed OpenAPI snapshot, including the GPT Image 2
names for JSON image editing.

## (manual)

Resolved `.castiron.stats.yml` conflicts while incorporating [the
earlier public
baseline](https://github.com/openai/openai-python/commit/5ac1e03e1ae8f08acf6213b3e44772734dbb8e59)
and, subsequently, [the current public
baseline](https://github.com/openai/openai-python/commit/9d3ba20f9567a62b2ebb3542661a60dc4ed2fd67).
Each intermediate merge retained the complete baseline record; CastIron
regenerated and published the final metadata. The latest rebase
preserves the test cleanups in
[#3697](https://github.com/openai/openai-python/pull/3697) and
[#3698](https://github.com/openai/openai-python/pull/3698): the full
`tests/` tree matches current `main` byte-for-byte. Full SDK formatting
and `git diff --check` passed. No handwritten runtime code changed.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>