MeasureManyStream never terminates (returns None)

[ThinkerDreamer]

Description

MeasureManyStream never returns None to signal stream completion. After all ping requests have been sent and all responses received, poll_next_unpin returns Poll::Pending forever instead of Poll::Ready(None).

This causes while let loops to hang indefinitely:

let mut stream = pinger.measure_many([localhost].into_iter());

// This hangs forever after receiving the ping response
while let Some((addr, rtt)) = stream.next().await {
    println!("{addr}: {rtt:?}");
}
// Never reaches here

Root Cause

In pinger.rs, poll_next_unpin has return type Poll<(V, Duration)> (no Option!) and always returns Poll::Pending at the end:

pub fn poll_next_unpin(&mut self, cx: &mut Context<'_>) -> Poll<(V, Duration)> {
    // ... process results ...
    Poll::Pending  // Always pending, never signals completion
}

The Stream implementation then wraps everything in Some, so it can never return None:

fn poll_next(...) -> Poll<Option<Self::Item>> {
    let result = ready!(self.poll_next_unpin(cx));
    Poll::Ready(Some(result))  // Always Some!
}

Expected Behavior

The stream should return None when:

1. All addresses in send_queue have been sent
2. All entries in in_flight have received responses (or timed out)

Suggested Fix

1. Change poll_next_unpin return type to Poll<Option<(V, Duration)>>
2. Add completion check: if send_queue.peek().is_none() && in_flight.is_empty() { return Poll::Ready(None); }
3. Update Stream impl to pass through the Option directly
4. Same changes needed for DualstackMeasureManyStream