Fix solverGenericCEGIS and make it also return the last failing cex

[lsrcz]

Previously this cex is ignored. We should also return them.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 54.93%. Comparing base (b87fd37) to head (8e5e107).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...c/Grisette/Internal/Core/Data/Class/CEGISSolver.hs	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #256      +/-   ##
==========================================
- Coverage   55.01%   54.93%   -0.08%     
==========================================
  Files         121      121              
  Lines       12801    12801              
  Branches      909      909              
==========================================
- Hits         7042     7032      -10     
- Misses       4850     4860      +10     
  Partials      909      909              

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