fix: recheck queue state before emitting deferred drain

[agammann]

Pushing an empty array schedules a drain event. If actual work is added before that event runs, the scheduled callback still fires and await q.drain() resolves while tasks are pending.

Recheck q.idle() inside the deferred callback. Add regression tests for push, unshift, pushAsync, and unshiftAsync, each followed by real work before the scheduled drain runs. This reproduces the behavior reported in #1729 without mixing async workers and callbacks.

Validation on Node.js 24.19.0: all four new tests fail before the fix; all 694 Node tests pass afterward. ESLint passes for both changed files.