write_text_stream() in the new stream.rs doesn't touch anything specific
to write_report (topic, findings, the agent) — it just drains a
MultiTurnStreamItem stream, writes each text chunk to a caller-provided
writer, and returns the accumulated string. Pulling it out lets it be
covered by unit tests against a mocked stream and an in-memory writer,
independent of a live model.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
