[GeoMechanicsApplication] Implement the formulations in the code (Extend Mohr-Coulomb)

[mnabideltares]

Summary

Extend the existing Mohr–Coulomb constitutive model to support hardening and softening (strain-dependent evolution of cohesion c, friction angle φ, and/or dilation ψ). Implement the necessary constitutive update (return mapping) and the consistent tangent operator.

Background / motivation

Currently the Mohr–Coulomb implementation uses fixed strength parameters. To model realistic geomaterials we must support evolution laws (hardening/softening) for cohesion and friction/dilatancy as functions of plastic strain (or other internal variables). This is required for simulations of localization, peak/softening, and more realistic failure patterns.

Requirements / acceptance criteria

• The model supports at least one common hardening/softening law (e.g. linear/exponential evolution of cohesion vs accumulated plastic strain α), and an easy way to add other laws.
• Plastic corrector uses an implicit return-mapping algorithm (stress update that enforces yield = 0 after plastic step).
• Compute/update internal variable(s): accumulated plastic strain α (and optionally others).
• Provide consistent algorithmic tangent (Jacobian) for the Newton solver.

Note that this issue should include unit tests.