buildRequestBody emitted prompt_cache_key and prompt_cache_retention for
every Codex request. The ChatGPT subscription OAuth backend
(chatgpt.com/backend-api) rejects prompt_cache_retention with HTTP 400
"Unsupported parameter", so every LLM call from an agent using a ChatGPT
OAuth Codex provider failed at the first think step.
Gate both params on isOpenAINativeEndpoint(apiBase), the same native-only
policy already documented and applied by CacheMiddleware. This covers both
CodexProvider and CodexAdapter, since the adapter delegates to the same
builder. Server-side prefix caching still works on the OAuth backend
without these params.
Update the two prompt-cache tests to assert both cases: native endpoints
include the params, the OAuth backend omits them.