Add codegen for BVProblems with cost functions

[SebastianM-C]

Co-Authored-By: Claude Opus 4.5 [email protected]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This is needed for SciML/BoundaryValueDiffEq.jl#413

[AayushSabharwal]

This needs a test

[SebastianM-C]

The Lotka-Volterra and the Cost function compilation from InterfaceII pass locally with SciML/BoundaryValueDiffEq.jl#413
After that PR is merged and we have a release, I can add more tests here.