OpenAI's Codex endpoint reports an exhausted ChatGPT subscription with
HTTP 429 and `error.type = "usage_limit_reached"`, but no `error.code`.
The Responses parser discarded the type and classified the response from
its status alone, making it appear to be transient request throttling.

This preserves `error.type` in both Responses and Chat Completions
parsing, considers both OpenAI discriminator fields during
classification, and falls back from the more-specific `error.code` to
`error.type` when populating the existing provider rejection code.
Usage-limit errors retain OpenAI's message and are not retried as
transient 429s.

The regression coverage exercises initial HTTP failures, Responses
stream errors, Chat Completions stream errors, unknown OpenAI-compatible
provider types, and precedence when both code and type are present.

Release Notes:

- Fixed ChatGPT subscription usage limits being shown as temporary
OpenAI request throttling.