The helper never waits on the runtime-tasks gather after cancelling it
(its children are bounded individually), so the finished-gather test must
hand the helper an already-complete gather to exercise the bounded
retrieval path, and the cancelled-gather test must settle the gather
itself instead of expecting the helper to await a still-pending future.
Use a pre-completed child for the finished case and suppress(await) for
the cancelled case; both now assert done() and a single close.
Covers the lifecycle contract of _close_gateway_runtime: runtime tasks are
cancelled before shared resources close, pending background work is drained
before the close returns, cancellation-swallowing tasks and hanging cleanup
are bounded by their timeouts, a failing close is logged without blocking the
stop, duplicate cleanup is idempotent, and the runtime_tasks gather await path
is exercised for both completed and cancelled gathers.
