Add support for the HiGHS optimization solver

[Copilot]

This PR adds complete support for the HiGHS optimization solver to RAMS, allowing users to leverage this high-performance linear and mixed-integer programming solver.

Changes Made

Core Implementation

• New solver class: Added lib/rams/solvers/highs.rb implementing the HiGHS interface following the existing RAMS solver pattern
• Model integration: Updated lib/rams/model.rb to include HiGHS in the available solvers list
• Output parsing: Implemented proper parsing of HiGHS solution format including:
  • Solution status (optimal, feasible, infeasible, unbounded)
  • Objective values
  • Primal variable values
  • Dual constraint values

Testing & Documentation

• Test coverage: Added comprehensive tests for HiGHS solver path customization in tests/test_solver_paths.rb
• Documentation: Updated docs/03-solver-configuration.md with HiGHS examples and configuration options
• Environment variables: Added support for RAMS_SOLVER_PATH_HIGHS to customize HiGHS executable path

Usage

Users can now use HiGHS by setting the solver:

m = RAMS::Model.new
# ... define variables and constraints ...
m.solver = :highs
m.solve  # Uses HiGHS solver

Custom Arguments

HiGHS-specific arguments can be passed through:

m.solver = :highs
m.args = ['--time_limit', '10', '--solver', 'simplex']
m.solve

Custom Installation Path

For non-standard HiGHS installations:

export RAMS_SOLVER_PATH_HIGHS=/opt/highs/bin/highs

Compatibility

• Supports both continuous and integer/binary optimization problems
• Maintains full compatibility with existing RAMS functionality
• All existing tests continue to pass
• Follows the same interface pattern as other RAMS solvers (CBC, CLP, CPLEX, GLPK, SCIP)

Fixes #33.

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[ryanjoneil]

Code looks good. Let's make sure it works in GitHub Actions.

• Add coin-or-HiGHS to the list of packages to install via dnf.
• Create and set an environment variable for running HiGHS tests in the workflow (RAMS_TEST_HIGHS: true).
• Add HiGHS to the tests in test/test_model.rb.

[Copilot]

Code looks good. Let's make sure it works in GitHub Actions.

• Add coin-or-HiGHS to the list of packages to install via dnf.
  ...

All requested changes implemented in commit bfe6668:

1. Added coin-or-HiGHS to dnf package installation in GitHub Actions workflow
2. Added RAMS_TEST_HIGHS: true environment variable to enable HiGHS testing
3. Added HiGHS solver tests to all test methods in tests/test_model.rb (simple, binary, integer, infeasible, unbounded, and implication tests)

HiGHS should now work properly in GitHub Actions CI/CD pipeline.

[ryanjoneil]

Removes CPLEX since we can't test it and want to focus on OSS solvers.