Fix pipeline stage check and add tests

[aobolensk]

• Avoid invalid iterators in IsFullPipelineStage
• Test premature PostProcessing calls

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.15%. Comparing base (0cdc845) to head (d808d62).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
modules/core/task/include/task.hpp	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   92.41%   92.15%   -0.26%     
==========================================
  Files          11       11              
  Lines         356      357       +1     
  Branches      125      127       +2     
==========================================
  Hits          329      329              
  Misses         10       10              
- Partials       17       18       +1     

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