Builds session.update with turn_detection: null on handshake (S4,
encoded in the translator's build_openai_session_update; Task 4).
Runs a select! loop over tap-side input (audio_in → append,
function_call_output → conversation.item.create) and OpenAI-side
input (response.audio.delta → tap audio_out, speech_started/stopped
→ tap speech_started/stopped, function_call_arguments.done → tap
function_call). 401 surfaces as OpenAiClientError::AuthFailed.
URL + headers shape tested directly. Full pump loop tested via
MockRealtimeBrain in Task 10 — neither side of this pump is a real
network endpoint in unit tests.
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