fix(nzb-dispatch): bound worker shutdown to one shared deadline (ported from fork)

[thedancingdeveloper]

Summary

Ports ec279d7 from the MrVampy/rustnzb fork: a correctness fix for WorkerPool shutdown.

The bug (still present on main)

WorkerPool::shutdown gives each worker handle its own 10s tokio::time::timeout:

let timeout = Duration::from_secs(10);
for h in handles {
    let _ = tokio::time::timeout(timeout, h).await;
}

So worst-case shutdown time scales with worker count — N workers that all hang ⇒ up to N×10s before the pool gives up.

The fix

A single shared deadline via timeout_at, after which any still-running handles are aborted, bounding total shutdown to ~10s regardless of worker count:

async fn await_worker_shutdown(mut handles: Vec<JoinHandle<()>>, timeout: Duration) {
    let deadline = tokio::time::Instant::now() + timeout;
    for handle in &mut handles {
        if tokio::time::timeout_at(deadline, &mut *handle).await.is_err() { break; }
    }
    for handle in handles {
        if !handle.is_finished() { handle.abort(); }
    }
}

Tests

Ships a #[tokio::test(start_paused = true)] test, worker_shutdown_uses_one_shared_deadline, that spawns 3 never-completing workers and asserts the shutdown is still pending at 9s and completes once the single 10s deadline passes (proving it does not wait 3×10s).

• cargo test -p nzb-dispatch → 42 passed
• cargo clippy -p nzb-dispatch --all-targets, cargo fmt --check → clean

Applied cleanly onto main (pre-image matched byte-for-byte; JoinHandle already in scope).

Original author: @MrVampy (commit preserved via cherry-pick -x).

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