debug: default terminal constraint softness parameters c_x̂min/max equal to 1

[franckgaga]

This is what is written in the documentation and the logical default, since a terminal constraint can lead to infeasibility. I also added a test that verify the default constraint values.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.55%. Comparing base (51cc8ad) to head (f0186d7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #299   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files          28       28           
  Lines        4980     4980           
=======================================
  Hits         4908     4908           
  Misses         72       72           

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