Fix 3140

[fwesselm]

Description

• Fix Regression in dual-fixing presolve v1.14.0 -> v.1.15.1 #3140 by re-setting flag for single equation handling done by HPresolve::dualFixing properly.
• I also ran the problematic models from Increased presolve time for LP problems in v1.14.0 #3022 to verify that there is no performance regression.
• A test was also added.

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)

[Opt-Mucca]

Short and sweet. Nice fix! Very nice find too @slowjazz

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.20%. Comparing base (f3a6291) to head (c814f2f).
⚠️ Report is 40 commits behind head on latest.

Additional details and impacted files

@@            Coverage Diff             @@
##           latest    #3232      +/-   ##
==========================================
- Coverage   73.21%   73.20%   -0.02%     
==========================================
  Files         445      445              
  Lines      107890   107981      +91     
  Branches    17278    17308      +30     
==========================================
+ Hits        78990    79043      +53     
- Misses      28624    28662      +38     
  Partials      276      276              

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