chore(api): Clarify image background docs and preview support (#3703)

## Summary

Refine documentation for the image 'background' parameter across the
Images resource and related type definitions. These changes only update
descriptions and wording; there are no changes to function signatures,
types, or runtime behavior.

## Changes

- Rewrite and clarify the 'background' parameter description for image
generation and editing to state it must be one of 'transparent',
'opaque', or 'auto' and that 'auto' lets the model choose the
background.
- Document that transparent backgrounds are available for supported GPT
Image models and explicitly mark gpt-image-2 and gpt-image-2-2026-04-21
support as preview-level, rather than stating transparent backgrounds
are not supported.
- Add/clarify guidance that when requesting a transparent background the
returned image format should support transparency (e.g., 'png' or
'webp').
- Apply these documentation updates consistently in runtime resource
docstrings and in typed models/typed dicts describing image parameters
and responses.
- No changes to public types, method signatures, parameter names,
accepted values, or return types; no customer-visible behavior or API
contract changes.

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