Increase MIP code coverage

[fwesselm]

Description

Refactored MIP code to increase code coverage

Checklist

• I have read the contributing guidelines
• This PR targets the latest branch
• Tests are passing
• Documentation was updated where relevant
• This PR is not primarily AI-generated (per the AI contributions policy in CONTRIBUTING.md)

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.21%. Comparing base (819a538) to head (bca0d0c).
⚠️ Report is 25 commits behind head on latest.

Files with missing lines	Patch %	Lines
highs/mip/HighsSearch.cpp	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3230      +/-   ##
==========================================
- Coverage   73.22%   73.21%   -0.01%     
==========================================
  Files         445      445              
  Lines      107826   107818       -8     
  Branches    17262    17271       +9     
==========================================
- Hits        78951    78940      -11     
- Misses      28599    28602       +3     
  Partials      276      276              

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[Opt-Mucca]

Great changes! LGTM.