Extracts the inner agent-streaming driver previously inlined as
_stream_agent_events in stream_new_chat.py.
stream_agent_events drives graph_stream.event_stream.stream_output and,
after the agent finishes, performs the post-stream safety-net work:
* commit any pending content the agent never explicitly finished
* evaluate file-operation contract outcomes and emit the appropriate
contract verdict for desktop_local_folder turns
This unit is what flows/shared/stream_loop.py wraps in the rate-limit
recovery while-loop. Add-only; no existing wiring uses it yet.
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