fix(harness): emit subagent end before parent completion (#2544)

## Description

### Background

The Harness stream-event tests can intermittently miss the sourced child
`AGENT_END` event. The child end event was emitted from `doFinally`,
which runs after the terminal signal has propagated downstream. For a
quickly completing child agent, the parent can therefore complete and
close its event sink before the child end event is forwarded.