Fix: Flux.zip does not emit errors from concurrent sources

[georgebanasios]

Fixes #3917

A race condition existed where a concurrent onComplete signal from one source and an onError signal from another could result in the onError signal being dropped.

Solution:

1. Synchronized Inner Subscriber State: The ZipCoordinator.error() method now takes full ownership of the termination sequence. It guarantees that the central error field is set before the inner subscriber's done flag is marked. This closes a timing window where the drain loop could previously see a done subscriber and mistakenly assume a completion.

2. Atomic Terminal State Tie-Breaker: Enhanced the drain() loop logic to use Exceptions.TERMINATED as an atomic sentinel value. This acts as a definitive tie-breaker for racing terminal signals. We ensure that only the first signal (either an error or a completion) can claim the terminal state, with any subsequent signals being correctly handled or dropped.