Don't add extra slacks (artificial variables) for ranged rows

[chris-maes]

When we have a ranged row of the form

l_i <= a_i^T x <= u_i

we convert it to an equality constraint for dual simplex by adding a slack s_i

a_i^T x - s_i = 0, l_i <= s_i <= u_i

We were also adding an additional slack variable for every equality constraint to ensure A was full row rank and we had a slack basis. For ranged rows this was leading to

a_i^T x - s_i + s_k = 0, l_i <= s_i <= u_i, 0 <= s_k <= 0.

This PR makes it so we no longer add s_k to rows that came from ranged rows.

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[rg20]

@chris-maes should this be merged?

[anandhkb]

This PR is NOT mandatory for 25.10 based on the Engine Lead's advise. We will still keep this marked for 25.10 aspirationally.

[chris-maes]

This is part of @hlinsen #602 PR. If that PR is going to be merged for 25.12 we can abandon this PR. If not, I can try to merge this for 25.12.

[anandhkb]

This PR is part of Hugo’s existing/open PR. If Hugo’s PR goes in to 25.12 we can close this one.