48d63d4ddf
Two TS errors in CI:
1. `agent-session.ts:1317` — `error TS1345: An expression of type 'void'
cannot be tested for truthiness`. The listener type is
`(event: AgentSessionEvent) => void`, so the returned value can't be
directly tested. Same shape in `agent.ts:1079`.
2. `test/session/emit-listener-isolation.test.ts:19` — the test fixture
for `AgentEvent.tool_execution_start` was missing the required `args`
field.
Cast the return to `unknown` and check `instanceof Promise` instead of
duck-typing `.then` — type-safe and matches what async functions actually
return. Add `args: {}` to the test fixture.