Streaming support: async generators and wrap(stream=True)

[tishachawla-jg]

Problem

0.2.0 supports async def boundaries but not async generators (SSE streaming). And wrap(client) does not handle stream=True (a streamed response is an iterator, not one object).

Proposal

• @boundary on an async def that yields: pass items straight through, record the assembled stream when exhausted, record a failed envelope on early close / error.
• wrap(..., stream=True): tee the stream, record the concatenated completion at the end; on replay, yield the recorded chunks.

Acceptance

• An async-generator boundary records and replays.
• A streamed wrap call records the full completion and replays the chunks.
• Tests for both.