Determine time spent in solver for CPLEX

[danielelerede-oet]

Changes proposed in this Pull Request

When calling CPLEX through linopy, the returned Result object contained only:

• status
• solution
• solver_model = m (the raw cplex.Cplex object)
  but it did NOT include the solver runtime.

This creates of course an issue for benchmarking cplex as we cannot reliably access the solve time from linopy, and of course the profiler misses it completely

Other solvers in Linopy already expose structured metrics:

Knitroreturns a KnitroResult with runtime + gaps + errors
HiGHS runtime accessible via solver info
Gurobi / Xpress can access runtime from API

So I'm proposing here to add a lightweight wrapper, similar to Knitro:

CplexResult = namedtuple("CplexResult", ["model", "reported_runtime"])
and modify the solve steps accordingly.

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.