* fix(ai): upgrade openai SDK to 7.19.0
Adds the "fast" service tier to the SDK types, needed to price
GPT-6 Fast mode requests correctly. Drops the local
prompt_cache_options type, which the SDK now defines.
* fix(coding-agent): check Cloudflare compat request via fetch instead of SDK mock
pi-ai now resolves its own nested openai 7.x while evals keeps 6.x at the
root, so vi.mock("openai") in coding-agent no longer reached pi-ai's client.
Capture the outgoing request with a fake fetch and assert the URL and headers.
* fix(ai): drop any casts for stream_options and max_tokens in openai-completions
stream_options is typed by the SDK, so assign it directly. max_tokens is
deprecated by OpenAI but still needed for OpenAI-compatible providers
that reject max_completion_tokens; use a narrow cast instead of any to
avoid the deprecation warning.
Move provider auth and OAuth flows onto pi-ai Models, compose models.json and extension overlays through ModelRuntime, and retain ModelRegistry as an extension compatibility facade.
