Locks the contract that a non-retryable failure (a Cloudflare 403
"managed challenge" page) returns a short, HTML-free `error` field —
guarding the field path where the raw page was dumped to Discord as
~31 messages.
The test drives the standard chat-completions path with a concrete
model so the turn actually reaches `client.chat.completions.create`,
where the mocked 403 is raised. It asserts the create call happened
(guarding against a vacuous pass — an empty model on the Codex
Responses path would otherwise abort on a validation ValueError before
any API call) and that the summarized error includes "403" while
excluding <html> / _cf_chl_opt. The non-retryable abort path is
provider-agnostic; a Cloudflare managed-challenge 403 can surface on
any provider behind Cloudflare.
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