Supports typed ResultMessage taxonomy overview

The agent SDK loop emits a typed terminal `ResultMessage` whose subtype is one of: success, error_max_turns, error_max_budget_usd, error_during_execution, error_max_structured_output_retries. Pairs with the `max_turns` / `max_budget_usd` loop knobs documented in https://docs.claude.com/en/api/agent-sdk/agent-loop.