Add shell-based HiGHS solver interface

[KasimVali2207]

Fixes #3894

Summary

The existing highspy-based HiGHS interface builds the problem in memory by iterating over variables and constraints in Python, which becomes a bottleneck for large models. This PR adds a file-based shell interface that avoids this overhead entirely.

Changes

New: pyomo/solvers/plugins/solvers/HIGHS.py

• HIGHS: factory entry point registered as SolverFactory('highs'). Dispatches to the shell interface by default, or to the persistent highspy backend when solver_io='direct'.
• HIGHSSHELL: a SystemCallSolver that writes an LP or MPS file, invokes the highs executable, and parses the solution and log files. Supports LP, MPS, duals, reduced costs, and all standard termination statuses (optimal, infeasible, unbounded, time limit, iteration limit).

Modified: pyomo/contrib/solver/plugins.py

• The persistent highspy interface is now registered as highs_persistent (legacy name highs_persistent_v2), consistent with how Pyomo handles cplex/cplex_persistent and gurobi/gurobi_persistent.

New: pyomo/solvers/plugins/solvers/tests/test_highs_shell.py

• 23 tests covering factory dispatch, command-line construction, log-file parsing for all statuses, solution-file parsing, and integration round-trips.

Usage

# Default — shell interface (LP file, highs executable)
opt = SolverFactory('highs')
opt.solve(model)

# MPS format
opt = SolverFactory('highs', solver_io='mps')

# Original persistent highspy interface
opt = SolverFactory('highs', solver_io='direct')
# or
opt = SolverFactory('highs_persistent')

Testing

python -m pytest pyomo/solvers/plugins/solvers/tests/test_highs_shell.py -v
# 23 passed, 5 skipped (skipped tests require highs binary on PATH)