fn parse_usage_event(parsed: &Value) -> Option<LlmStreamEvent>Expand description
Parse a top-level usage object into a LlmStreamEvent::Usage event.
Returns None when the frame carries no usage field. Deliberately
returns just the event (not a full SseParseResult) — unlike
parse_inline_error_frame, a usage field does not imply the rest
of the frame should be skipped. See the call site in parse_sse_frame
for why.