Refactor Osi solver creation and enable MibSModel extensions

[hlefebvr]

Hi,

Description

This PR refactors all locations where an external Osi solver is created for bilevel feasibility checks or auxiliary problem solving. The solver creation logic is now centralized in a new method:

MibSModel::createFeasCheckSolver()

which returns a pointer to an OsiSolverInterface.

Changes

• Centralize Osi solver creation into MibSModel::createFeasCheckSolver()
• Make MibSModel extensible:
  • Mark the constructor as virtual
  • Mark createFeasCheckSolver() as virtual

Motivation

This change allows users to override the solver creation mechanism and plug in any OsiSolverInterface implementation, beyond those currently supported by MibS.

Impact

• No change to existing behavior
• Improves code organization and reduces duplication
• Enables user-defined extensions via inheritance

Notes

This PR is purely a refactor with added extensibility and does not alter the current solving workflow.

Thanks,

Henri

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