Extract _build_partial_message() shared by cancel_stream() and the new
_snapshot_and_append_partial_on_error(); the two provider/exception error paths
in _run_agent_streaming now snapshot accumulated text/reasoning/tool_calls under
STREAMS_LOCK and append a _partial assistant message instead of discarding it.
Closes#3929.
Co-authored-by: b3nw <150195942+b3nw@users.noreply.github.com>
