fix: compute a single IIS in Xpress infeasibility path

[FabianHofmann]

Changes proposed in this Pull Request

• Model._compute_infeasibilities_xpress now computes a single IIS via firstIIS(2) (new Xpress API) with iisfirst(2) as deprecated-API fallback, instead of calling iisall() and iterating every IIS.
• The previous code merged constraints from all IIS into one deduplicated set, flattening per-IIS grouping and returning an ambiguous union. With only one IIS computed, the returned list[int] matches the Gurobi path's semantics.
• Mode 2 asks Xpress for a proper IIS optimised for a quick result (mode 0 would return a non-minimal infeasible subproblem; mode 1 optimises for minimality at higher cost).
• Release note added under Upcoming Version.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

would like to make a patch release with this and #605