SESSION_COST_EVENT_TYPE = 'session_cost'Custom-stream event type carrying the thread's absolute cumulative cost.
Emitted by the durable writer so the status bar can track spend live without
re-pricing anything. The payload is {"type": ..., "total": <usd>, "thread_id": <id>, "pricing_ok": <bool>}; total is the full thread lifetime estimate,
never a delta, so a client that misses an event still converges on the next one.
thread_id lets a client that has since switched threads discard a total
belonging to the previous one. pricing_ok reports whether price data loaded in
the process that actually did the pricing, which is the only way a client can
tell a broken remote install from models with no published rates.