When gpt-image upstream returns response.completed with no image but a text
refusal (content moderation, e.g. the model replies "this request was judged
unsafe to generate"), the soft-failure path treated it as a probabilistic
upstream failure and returned a retryable UpstreamFailoverError (502). Retrying
or switching accounts is futile for a content-policy block — it just burns
other accounts' quota and still surfaces an opaque 502 to the client.
Distinguish the two no-image cases:
(A) model text refusal -> 400 content_policy_violation, no retry, refusal
reason passed through to the client.
(B) truly empty response -> unchanged: retryable UpstreamFailoverError (502).
Adds extractOpenAIImagesModelRefusal (extracts the refusal text from
output_text.delta / message output_text, capped at 600 chars) and two unit
tests; the existing empty-response retry test is unaffected.
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