Apply optimizations:
* https://github.com/openai/openai-go/pull/602
* https://github.com/coder/aibridge/pull/160
These reduce CPU time and allocation count for OpenAI `chat/completions`
and `responses` APIs, making the use of OpenAI chat models through AI
Bridge more performant.
In order to test these changes, we add scaletesting support for the
responses API.