Same mechanism as for BYOK: `service_tier == priority`. Most of the work
is already done. When validating this in manual testing, I noticed we
get back `service_tier == auto` in the response, unlike in the regular
OpenAI API scenario with BYOK, but apparently [it doesn't mean priority
tier wasn't
applied](https://github.com/openai/codex/issues/14204#issuecomment-4033184620).
It's not a hard confirmation, but the model does seem to respond faster
when I toggle fast mode on.

Release Notes:

- Added Fast Mode (priority service tier) support to OpenAI models used
through the ChatGPT subscription provider.