Fix DiffusionGrid::GetConcentration error label

[stanbot8]

Summary

• The out-of-bounds error in GetConcentration incorrectly identifies itself as ChangeConcentrationBy, making it impossible to distinguish read vs write OOB errors when debugging.

Test plan

• Existing unit tests pass (no behavioral change, label only)

[sportokalidis]

Hi @stanbot8,

Thanks for the contribution! I’ll review the changes and get back to you.

In the meantime: the CI issue that previously prevented macos-system-ci and ubuntu-system-ci from running has been fixed on main.

Could you please update your PR branch with the latest main (merge main into your branch, or rebase onto main) and push? That should trigger the missing system CI workflows on this PR.