fix: race condition on cancellation in UniCallbackSubscriber

[jponge]

Fixes #1494

[jponge]

/cc @DavideD

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dded2d6) 89.34% compared to head (be54f15) 89.13%.
Report is 6 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1495      +/-   ##
============================================
- Coverage     89.34%   89.13%   -0.21%     
+ Complexity     3365     3355      -10     
============================================
  Files           459      459              
  Lines         13434    13435       +1     
  Branches       1635     1636       +1     
============================================
- Hits          12002    11975      -27     
- Misses          808      821      +13     
- Partials        624      639      +15     

Files	Coverage Δ
...smallrye/mutiny/helpers/UniCallbackSubscriber.java	84.61% <100.00%> (+15.38%)	⬆️
...llrye/mutiny/operators/multi/MultiConcatMapOp.java	87.41% <100.00%> (+0.08%)	⬆️

... and 11 files with indirect coverage changes

[DavideD]

That was quick, thanks