Review feedback (round 2). Narrowing the handler in the previous commit
fixed the silent-pass but reintroduced #1947's own failure class on the
unexpected path: `then` returns a *derived* promise, and the re-throw
rejects that one rather than `call`. The caller doesn't await it until
after `disconnect()`, so a rejection arriving while the test is still
waiting on progress notifications sits unobserved for seconds and Node
reports it as an unhandled rejection.
Attach an observer to the derived promise the moment it exists. Nothing is
swallowed — it stays rejected, so the caller's `await` still fails the test.
Verified both directions with a probe rejecting with a non-ConnectionClosed
error and awaiting after a 1.5s gap: without the observer the run reports
"Unhandled Rejection" + "Errors 1 error"; with it, the test fails and the
run stays clean.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HDdo1rNRVQnVRRdsqSrvG3
c60563b7d6