Maps the existing `Speed::Fast` plumbing to OpenAI's `service_tier:
"priority"`, which matches what "fast mode" in Codex does. Relevant docs
[here](https://platform.openai.com/docs/api-reference/chat/create#chat-create-service_tier).

Like for the existing Anthropic fast mode we have a
`Model::supports_priority` method for the variants on
https://openai.com/api-priority-processing. Pro, nano, and legacy gpt-4
are excluded; Custom defaults to false.

This is gated to staff only for now (not in this diff, but the existing
fast mode feature), until we have the mechanism to require confirmation
before you enable fast mode.

Release Notes:

- Added support for Fast Mode (priority service tier) on the OpenAI API
provider.