Don't eliminate free variables in dual simplex

[chris-maes]

Fixes an issue where free variables were being eliminated in dual simplex. These variables should only be eliminated when using barrier.

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[chris-maes]

/ok to test 6726254

[ramakrishnap-nv]

@chris-maes PR is failing with following test failures https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52175602693?pr=457#step:10:3845
https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52175602687?pr=457#step:10:4559

FAILED tests/test_pdlp_warmstart.py::test_warmstart - assert (36800 + 104480) == 136280

https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52174832887?pr=457#step:10:2140
[ FAILED ] c_api/TimeLimitTestFixture.time_limit/1, where GetParam() = ("/linear_programming/square41/square41.mps", 5, 1)

[chris-maes]

@chris-maes PR is failing with following test failures https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52175602693?pr=457#step:10:3845 https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52175602687?pr=457#step:10:4559

FAILED tests/test_pdlp_warmstart.py::test_warmstart - assert (36800 + 104480) == 136280

https://github.com/NVIDIA/cuopt/actions/runs/18319680798/job/52174832887?pr=457#step:10:2140 [ FAILED ] c_api/TimeLimitTestFixture.time_limit/1, where GetParam() = ("/linear_programming/square41/square41.mps", 5, 1)

@rgsl888prabhu that is strange. This commit should only affect dual simplex and barrier. It would not affect PDLP warm start or a MIP solve time test. I re-triggered the tests. Let's see if they fail again.

[chris-maes]

/merge