Fix: handle HighsModelStatus.kSolutionLimit like kIterationLimit

[rschwarz]

Was previously not handled at all, resulting in termination_condition of unknown, and no further information about whether a solution was found.

kIterationLimit may not be exact, but it's the best fit among the current enum values, I guess.

Fixes #3632 .

Summary/Motivation

handle HighsModelStatus.kSolutionLimit like kIterationLimit

Changes proposed in this PR:

• add case for kSolutionLimit
• add warning for the else branch

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[mrmundt]

So I understand - does kSolutionLimit exist for all versions of highspy, or is it only present in 1.5?

[rschwarz]

My understanding is that it was added to HiGHS v1.5, and has remained to the present day.

EDIT: See this HiGHS test which is still on the main branch:

https://github.com/ERGO-Code/HiGHS/blob/364c83a51e44ba6c27def9c8fc1a49b1daf5ad5c/check/TestMipSolver.cpp#L62-L67

[mrmundt]

Okay, then this is going to need a bit of work. This change would effectively pin us to a minimum version of highspy, and we try our best to maintain backwards compatibility as much as possible. Additionally, there are actually two different HiGHS interfaces - contrib/appsi and contrib/solver. We have a dev call tomorrow and will talk about it then. Thanks for submitting the PR! I'll let you know soon what we decide.

[rschwarz]

If it helps, I could add in an additional version check for HiGHS in the code.

[jsiirola]

A version check would be OK, but something like

       elif status == getattr(highspy.HighsModelStatus, "kSolutionLimit", NOTSET):
            results.termination_condition = TerminationCondition.maxIterations

might be more performant and simpler.

[rschwarz]

A version check would be OK, but something like

       elif status == getattr(highspy.HighsModelStatus, "kSolutionLimit", NOTSET):
            results.termination_condition = TerminationCondition.maxIterations

might be more performant and simpler.

This looks good to me, but I'm not sure if this might cause some type problems, as highspy.HighsModelStatus is actually an enum at the C++ level.

Please let me know if which changes I should apply, if any.

PS: I'm now aware of the non-APPSI interface to HiGHS, which is affected by the same problem.