Return self in model_validator for BoreschRestraintSettings

[IAlibay]

Fixes #2216

My understanding of the model_validator docs is that we should return self at the end. It looks like this is the cause of the issue (not tried it locally yet).

Relevant docs section:

After validators: run after the whole model has been validated. As such, they are defined as instance methods and can be seen as post-initialization hooks. Important note: the validated instance should be returned.After validators: run after the whole model has been validated. As such, they are defined as instance methods and can be seen as post-initialization hooks. Important note: the validated instance should be returned.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (1820817) to head (6480473).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2217      +/-   ##
==========================================
- Coverage   95.07%   90.33%   -4.74%     
==========================================
  Files         206      206              
  Lines       20697    20698       +1     
==========================================
- Hits        19677    18697     -980     
- Misses       1020     2001     +981     

Flag	Coverage Δ
fast-tests	90.33% <100.00%> (?)
slow-tests	?

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