Add CommonsMathSolver Adapter for AbstractLPModel

[xts-Michi]

Add CommonsMathSolver Adapter for AbstractLPModel

This PR introduces the first solver integration into the abstract interface.

Key Changes

CommonsMathSolver Integration

• Added CommonsMathSolver:

  • Accepts a finalized (build()-ed) AbstractLPModel and solves the linear program using Apache Commons Math's SimplexSolver
  • Extracts variables, bounds, constraints, and objective function directly from the model (no redundant mapping required)
  • Fully supports both maximization and minimization via the OptimizationDirection enum

• Standardized Result Handling:

  • Introduces or updates LPSolution as a single, extensible container for all solver results: variable assignments, objective value, and feasibility flag
  • Ensures results are accessible via variable names (Map<String, Double>)

Tests

• Added generic unit tests for the solver interface and adapter:
  • Abstract base tests (BaseLPSolverTest) ensure consistent behavior for any solver integration
  • Concrete test class (CommonsMathSolverTest) runs all standard LP tests against the new CommonsMathSolver
• The existing demo (SolverDemo) remains for manual/interactive exploration
• Test coverage includes:
  • Adding variables and constraints
  • Model build/finalization logic
  • Duplicate name handling
  • Enum-based maximize/minimize direction
  • Model mutability and debug flag

[matthiaskoenig]

minor comments, ready to merge