Turn events now include token usage data

What

• The turn.complete event payload now includes a computed usage object when available, alongside the existing answer, durationMs, aborted, and turnId fields.
• The turn.step and turn.complete telemetry event payloads now include a usage field whenever the underlying event carries usage data.
• The plugin/hooks event-summary logger now includes token usage details (input tokens, output tokens, cache-read tokens, cache-creation tokens, and the model name) on turn.step and turn.complete events, pulled from the assistant message.

Why

Plugins, hooks, and telemetry consumers can now see how many tokens a turn used directly from the turn events, without needing a separate lookup, which helps with cost tracking and observability.