The session event stream (api_server.py:~2236) was the one genuinely
unicode-distinct SSE writer — json.dumps(payload, ensure_ascii=False) +
.encode('utf-8'). Every other writer uses plain json.dumps. Route it
through _sse_frame(..., ensure_ascii=False) so _sse_frame is now the single
source of truth for ALL SSE frame serialization in the module (chat-
completion, responses._write_event, /v1/runs, and the session stream).
Byte-identical for non-ASCII payloads: verified against the historical
inline encoder (raw bytes preserved). The ensure_ascii=False path is now
exercised by test_sse_frame_ensure_ascii_false_reproduces_session_event_stream.
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