Confirms the existing proxy streams SSE chunks incrementally:
- 5-chunk test with 50ms delays verifies time spread between first
and last chunk arrival (not buffered)
- Verifies data: [DONE] terminator is forwarded
No src/ changes needed — Body::from_stream(bytes_stream()) already
handles SSE correctly.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
