fix(harness): subagent event stream never closes when the parent is cancelled (#2481)

Fixes #2480.

## Problem

`AgentSpawnTool` emits `AgentStartEvent` unconditionally before
subscribing to a sync-spawned child, then emits the matching
`AgentEndEvent` from `doOnTerminate`. `doOnTerminate` fires on
`onComplete` / `onError` but **not on cancel**, so a parent cancel (user
"Stop", outer `timeout()`, Reactor `dispose()`) leaves the child's event
stream open — consumers see a start with no end and keep rendering the
subagent as running.