Avoid stalling in HighsRedcostFixing::addRootRedcost

[fwesselm]

Description

• Avoid stalling in HighsRedcostFixing::addRootRedcost for huge bounds (>= kHighsIInf) on integer variables.
• Add a unit test.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.23%. Comparing base (d7c50d1) to head (5b23b52).
⚠️ Report is 8 commits behind head on latest.

Additional details and impacted files

@@           Coverage Diff           @@
##           latest    #3241   +/-   ##
=======================================
  Coverage   73.23%   73.23%           
=======================================
  Files         445      445           
  Lines      108019   108070   +51     
  Branches    17318    17321    +3     
=======================================
+ Hits        79106    79150   +44     
- Misses      28637    28644    +7     
  Partials      276      276           

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[jajhall]

I'll still add the code to transform bounds greater than or equal to kHighsIInf on integer variables to kHighsInf.

However, if someone gives an integer variable a bound of kHighsIInf-1, bound modifications in presolve could still lead to integer variables with a bound greater than kHighsIInf, so the safeguard in this PR is wise.

[fwesselm]

I'll still add the code to transform bounds greater than or equal to kHighsIInf on integer variables to kHighsInf.

However, if someone gives an integer variable a bound of kHighsIInf-1, bound modifications in presolve could still lead to integer variables with a bound greater than kHighsIInf, so the safeguard in this PR is wise.

I am going to push some more code changes to this PR to handle overflows if one or both bounds are extreme (e.g., kHighsIInf - 1).

[Opt-Mucca]

Strong support for the change!