Stop a running paper trading session. The stop path cancels only open orders durably owned by the session journal. Existing positions are always left open; this tool has no flatten or liquidation capability. If every cancellation cannot be confirmed, the session returns an actionable error instead of claiming a clean stop.

stop_paper_trading(session_id)   # → cancellation outcome; positions remain open

Errors: SESSION_NOT_FOUND if the session doesn't exist.