Add flexible solver strategies, solution modes and result normalization to Modello

[Code0x58]

Motivation

• Provide more flexible constraint solving for Modello instances to handle legacy, set-based and parametric solver outputs.
• Allow callers to control how non-unique solutions are handled via a solution_mode and a solution_selector to pick a solution when multiple exist.
• Normalize solver outputs into a structured result to simplify downstream attribute assignment and edge-case handling.

Description

• Introduce SolverStrategy, SolutionMode, and SolutionKind enums and a SolveResult dataclass to represent normalized solver outputs.
• Extend Modello.__init__ with solver_strategy, solution_mode, and solution_selector parameters and replace inline solve logic with a dedicated _solve_constraints method.
• Implement _solve_constraints which supports the legacy solve behavior and a set strategy that uses linsolve/nonlinsolve and falls back to per-symbol solveset for parametric results, and returns SolveResult objects.
• Update imports and add logic to select solutions, raise on multiple solutions in strict mode, and expose parametric solution sets when appropriate.

Testing

• Ran unit tests with pytest including new tests test_solver_unique_solution, test_solver_multiple_solution_permissive_and_selector, and test_solver_underdetermined_set_strategy, and all tests passed.
• Existing tests such as test_no_constraints and test_multiple_inheritance_expr_conflict were executed and continued to pass.

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Codex Task