feat: termination condition mapping for highs solver

[ollie-bell]

Closes # (if applicable).

Changes proposed in this Pull Request

Explicit mapping from highs model status to linopy termination conditions. Previously it was relying on calling .lower() on the output of h.modelStatusToString, but that only captures the optimal and infeasible cases.

See https://github.com/ERGO-Code/HiGHS/blob/fd8665394edfd096c4f847c4a6fbc187364ef474/src/lp_data/HighsModelUtils.cpp#L1330 for the outputs of h.modelStatusToString.

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]

@ollie-bell great work! thank you! @siddharth-krishna could you have a short look if that is affecting your benchmark results in any case?