SlowAPI middleware can short-circuit any request with 429 once a
per-route or per-IP rate limit fires (e.g. POST /api/v1/auth/login is
capped at 10/5min). The OpenAPI spec did not declare 429 on any
operation, so schemathesis flagged legitimate rate-limit responses as
RejectedPositiveData / status-code-nonconformance failures.
Override app.openapi to inject a generic 429 response object on every
HTTP operation in the generated schema. Add a contract test that fails
if any operation drops the 429 advertisement.