recordAssistantUsage accumulated cacheRead across the entire multi-turn
run, and totalTokens was clamped to contextTokens. This caused
session_status to report 100% context usage regardless of actual load.
Changes:
- run.ts: capture lastTurnTotal from the most recent model call and
inject it into the normalized usage before it reaches agentMeta.
- usage-reporting.test.ts: verify usage.total reflects current turn,
not accumulated total.
Fixes#17016
