feat: implement Robin boundary conditions (convection) for thermal optimization

[rainerrodrigues]

…timization

Summary

This PR introduces convective boundary conditions (Newton's Law of Cooling) to HeatConductionProblem, enabling more realistic thermal topology optimization (e.g., modeling heat sinks cooled by ambient air).

Highlights:

Surface Integral Assembly: Implemented assemble_convection_matrix using Ferrite's FaceScalarValues and QuadratureRule{dim-1} to evaluate the boundary integral for convection.

Zero-Cost Solver Injection: Updated the GenericFEASolver call operator to inject the constant convection matrix into the global stiffness matrix. This is done via in-place mutation (K_data[row, col] += val) to perfectly preserve Ferrite.jl's matrix pre-allocations and safely maintain the Symmetric type wrapper. Due to dispatch on Physics === HeatTransfer, this adds zero runtime overhead to structural problems.

Type-Stable Struct Expansion: Added a parameterized convectiondict field to HeatConductionProblem to map boundary names to their heat transfer coefficient and ambient temperature (h, T_inf). The constructor defaults to an empty dictionary, ensuring complete backwards compatibility with existing scripts.

Minor changes:

Added getconvectiondict fallback and specific getter methods.

Updated module exports in TopOptProblems.jl and imports in FEA.jl to properly expose the new assembly logic.

What type of change is this?
[ ] Bug fix in a backwards-compatible manner.

[x] New feature in a backwards-compatible manner.

[ ] Breaking change: bug fix or new feature that involve incompatible API changes.

[ ] Other (e.g. doc update, configuration, etc)

Checklist
[x] I ran all tests on my computer and it's all green (i.e. ] test).

[ ] I have added tests that prove my fix is effective or that my feature works.

[ ] I have added necessary documentation (if appropriate)