Replace the plugin-local _IMAGE_MAGIC_MIME table + _sniff_image_mime
body with a delegation to agent.image_routing._sniff_mime_from_bytes,
the canonical magic-byte sniffer already used across the codebase, then
gate its result to the raster formats gpt-image-2's Responses
input_image actually accepts (png/jpeg/gif/webp).
The shared sniffer also recognizes SVG/TIFF/ICO; without the allowlist
those would pass local validation and be rejected server-side with an
opaque HTTP 400. Gating locally fails them cleanly as invalid_image_input.
Adds a regression test for SVG rejection.
Follow-up on top of @CrazyBoyM's #55828.
New built-in image_gen backend at plugins/image_gen/openai-codex/ that
exposes the same gpt-image-2 low/medium/high tier catalog as the
existing 'openai' plugin, but routes generation through the ChatGPT/
Codex Responses image_generation tool path. Available whenever the user
has Codex OAuth signed in; no OPENAI_API_KEY required.
The two plugins are independent — users select between them via
'hermes tools' → Image Generation, and image_gen.provider in
config.yaml. The existing 'openai' (API-key) plugin is unchanged.
Reuses _read_codex_access_token() and _codex_cloudflare_headers() from
agent.auxiliary_client so token expiry / cred-pool / Cloudflare
originator handling stays in one place.
Inspired by #14047 by @Hygaard, but re-implemented as a separate
plugin instead of an in-place fork of the openai plugin.
Closes#11195
