test: add streaming coverage for OpenAI chat completions provider

Cover the stream() method's inner _produce() function which was
previously untested. Tests exercise text streaming, tool call
streaming, text-then-tool transitions, kwargs building (system
prompt, tools, on_response/on_payload hooks), abort signal
handling, error paths (Exception and BaseException), response ID
capture, empty-choices skipping, finish_reason mapping, and the
no-finish-reason fallback. Coverage goes from 29% to 100%.

Author: xfgong