This commit fixes an issue where a Cacheable method which returns a
Flux (or multi-value publisher) will be invoked once, but the returned
publisher is actually subscribed twice.
By using the Reactor `tap` operator, we ensure that we can emit values
downstream AND accumulate emitted values into the List with a single
subscription.
The SignalListener additionally handles scenarios involving cancel,
for instance in case of a `take(1)` in the chain. In that case values
emitted up until that point will have been stored into the List buffer,
so we can still put it in the cache. In case of error, no caching occurs
and the internal buffer is cleared. This implementation also protects
against competing onComplete/onError signals and cancel signals.
Closes gh-32370
988f3630c4