_is_structured_output_rejection matched several phrasings for a provider refusing the
structured-output field, but not gateways that validate the body with a strict pydantic
model and reject the OBJECT-form json_schema by shape:
HTTP 422: {"detail":[{"loc":["body","response_format","json_schema"],
"msg":"str type expected","type":"type_error.str"}]}
422 already passes the status check; only the phrasing was missing, so the
one-retry-without-response_format rung never engaged and the auxiliary task failed
hard (title generation left 'HTTP 422' in the session title). Treat the shape error as
a rejection: the field is what the provider refuses and the remedy is identical.
Fixes#110631
moxian
9c7de107f2