Contrib HiGHS: support partial MIP warm starts

[adbuerger]

Fixes

Part 2 of #4044 (part 1: #4046)

Summary/Motivation:

The contrib.solver HiGHS interface (highs) has no MIP warm start. The Gurobi and SCIP interfaces in the same framework provide the warmstart_discrete_vars option, which passes the current values of the integer variables to the solver. This PR adds the same option for HiGHS. Because only discrete values are passed, the start is partial by design. HiGHS supports partial starts since 1.8 through the sparse setSolution(num_entries, index, value) overload: all columns not passed are set to kHighsUndefined, and HiGHS completes the start itself.

Changes proposed in this PR:

• Add HighsConfig (derived from PersistentBranchAndBoundConfig) with the option warmstart_discrete_vars (default False), worded as in the Gurobi and SCIP interfaces.

• Add Highs._mipstart(), which passes all integer variables that have a value to HiGHS via the sparse setSolution overload. On HiGHS < 1.8, which has neither the overload nor partial starts, it logs a warning and skips the start. It is called before capture_output, because a warning logged inside that block would only end up in the solver log.

• Add tests to test_highs.py:

  • a full start, whose reported objective value also shows that continuous values are not passed;
  • a partial start;
  • two re-solves on the same persistent instance: the start uses the current values (including the loaded results of the previous solve), and a value reset to None is not carried over from the previous start;
  • the warning on HiGHS < 1.8, using a subclass that reports version 1.7.0 (no mock).

  The warm-start tests are skipped on HiGHS < 1.8.

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Notes for reviewers (optional):

Behaviour on re-solves: every solve passes the values the variables have at that time, so after a solve the loaded results serve as the next start. Variables without a value are left undefined, because a new sparse solution is passed on every solve. With the option off, nothing is passed, although HiGHS itself may reuse its last solution if the model has not changed (existing HiGHS behaviour, not touched here).

One question:

• Gurobi's _mipstart() only sets Start for variables that have a value. In the persistent interface, a variable reset to None would therefore probably (I did not verify it yet) keep its Start from an earlier solve. Is that intended?

The partial-start test uses the same deliberately loose constraint as in #4046 (x1 <= 3 + 7 * x3); with a tighter version, presolve solves the model before the start is evaluated. The sparse setSolution overload is available from HiGHS 1.8 on; I tested with highspy 1.15.1.

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