Result returned from a completed tool loop.
:usage is the token-usage totals summed across every stream_text/3
call made during the loop (one per iteration). Keys mirror what the
underlying ReqLLM provider reports — typically :input_tokens,
:output_tokens, and any cost/cache fields. Numeric fields are summed;
non-numeric fields fall through to the most recent value. Empty %{}
when no provider reported usage.