Codex Responses streams can emit partial_image_b64 previews without a final
image_generation_call.result. The provider treated any b64 as success and could
let a partial overwrite a coexisting final in the same payload, delivering
smeared intermediates as finished GPT Image 2 outputs.
Request partial_images=0, prefer final over partial in extraction, fail closed
(with one content-agnostic retry) unless source=final, and surface image_source
plus pixel_size for QA.
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