onFinish always receives a totalUsage object, so the `?? {}` guard and
optional chaining were dead. Extract the field-level extraction into a
recordTurnUsage method (totalTokens, else input+output) and unit-test that
recordShareTokens receives the summed value when totalTokens is absent and the
authoritative total when present.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>