[Test] SSE buffer boundary flush assertion too weak

[kianwoon]

Description

The SSE buffer boundary flush test ("size threshold: flushes at \\n\\n boundary when bufferBytes exceeded") only checks expect(enqueue.mock.calls.length).toBeGreaterThanOrEqual(1) — it doesn't verify:

1. The flushed chunk ends at a \n\n boundary
2. The unflushed remainder (partial event) is retained for the next flush

The boundary scan (stream-buffer.ts:57-63) scans backwards for \n\n. This is a critical correctness path — flushing mid-event would corrupt the SSE stream.

Location

• tests/stream-buffer.test.ts:36-48

Suggested Fix

Assert:

• Flushed chunk ends with \n\n
• Next flush contains the retained remainder