fix(api): preserve model choices and defer error response docs (#3931)

## Summary

Preserves legacy model choices and rolls back unreleased error-response
documentation without changing Python exception behavior.

## Changes

- Preserve gpt-5.1-mini and existing audio model choices; this does not
imply model availability.
- Remove the recently added error-status, error-envelope, and
retry-header documentation pending further review.
- Keep existing Python exception types and decoding behavior unchanged.

Co-authored-by: marcuswood-oai <323319819+marcuswood-oai@users.noreply.github.com>