Report concurrent onNext signals in HalfSerializer

[jponge]

Such concurrent signals are a violation of the Reactive Streams protocol.

[codecov]

Codecov Report

Merging #1144 (b621789) into main (2042453) will increase coverage by 0.07%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1144      +/-   ##
============================================
+ Coverage     89.68%   89.76%   +0.07%     
  Complexity     3348     3348              
============================================
  Files           458      458              
  Lines         13224    13227       +3     
  Branches       1646     1646              
============================================
+ Hits          11860    11873      +13     
+ Misses          709      699      -10     
  Partials        655      655              

Impacted Files	Coverage Δ
...ava/io/smallrye/mutiny/helpers/HalfSerializer.java	80.95% <100.00%> (-7.94%)	⬇️
...erators/multi/builders/SerializedMultiEmitter.java	79.26% <0.00%> (-6.10%)	⬇️
...mallrye/mutiny/operators/multi/FlatMapManager.java	95.83% <0.00%> (-2.78%)	⬇️
...ny/operators/uni/builders/UniCreateFromFuture.java	88.52% <0.00%> (-1.64%)	⬇️
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	89.36% <0.00%> (-1.00%)	⬇️
...a/io/smallrye/mutiny/helpers/BlockingIterable.java	86.11% <0.00%> (-0.93%)	⬇️
...lrye/mutiny/subscription/SerializedSubscriber.java	86.23% <0.00%> (+0.91%)	⬆️
...subscription/SwitchableSubscriptionSubscriber.java	95.23% <0.00%> (+1.58%)	⬆️
...java/io/smallrye/mutiny/helpers/Subscriptions.java	81.76% <0.00%> (+1.65%)	⬆️
... and 5 more