1247 Replace Uniquenes validation for xml_id in ModelSolution and Test

[kkoehn]

fixes #1247

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.81%. Comparing base (5a9d6d3) to head (9714a2b).
Report is 265 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1852      +/-   ##
==========================================
+ Coverage   94.79%   94.81%   +0.01%     
==========================================
  Files         133      133              
  Lines        3345     3353       +8     
==========================================
+ Hits         3171     3179       +8     
  Misses        174      174              

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[arkirchner]

Thank you!