fix(codex): surface malformed models-list bodies and merge var mismatch

transformModelsListResponse swallowed a malformed-JSON body behind a 200
status, presenting an upstream failure as an empty model list. Now
returns a 502 api_error instead.

Also fixes a stray reference to transformedRequestForRetry (a variable
from the PR's pre-#382 base, dropped by the merge's request-rebuild
resolution but left in two header reads) that would have thrown a
ReferenceError on any Codex in-place 529 retry.