Accept `include_evidence` on peer and workspace chat, sync and async. Opting
in returns a `ChatResponse` carrying the answer alongside what the dialectic
read to produce it; leaving it out returns the answer on its own, so existing
callers are unaffected.
Overloads discriminate on the flag's literal value, so a Pydantic
`response_format` combined with evidence types as `ChatResponse[Model]` while
the default call still types as `str | None`. The four chat methods shared
identical response-reading logic, which now lives in one `parse_chat_response`
so they cannot drift.
Evidence for a streamed answer can only be known once the answer is complete,
so the server sends it on the stream's terminal event. `SSEStreamParser` now
keeps it instead of discarding that event, and the stream response exposes it
as `evidence` once drained. Reading it mid-stream returns None.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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