feat(agui): add a shared reader for handler streams (#164)

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Handlers can yield the AG-UI text and tool subset. `config().stream()`
still yields `chunk` then `done`. `launchdarkly_ai_server.agui.stream`
is the opt-in reader: it brackets a run with `RUN_STARTED` and
`RUN_FINISHED`, and `capture_content` gates argument deltas and tool
results.

Verified with `pytest packages/client/tests` and `mypy
packages/client/src`.
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>

Author: apucacao