The openai-completions provider now calls .withResponse() on the
create() result to surface HTTP status/headers for the new onResponse
hook (d131fcd4). The FakeOpenAI test mock only returned an async
iterable, so 4 tests in openai-completions-tool-choice.test.ts that
actually consume the stream failed with "withResponse is not a function".
Updated the mock to return a Promise augmented with .withResponse()
resolving to { data: <async iterable>, response: { status, headers } }.
closes#3304
