MeshSurfaceFilter partial current labels

[tjlaboss]

Description

Renames tally filter bin labels for partial currents belonging to structured meshes under a MeshSurfaceFilter. Previously, OpenMC would show 'x', 'y', and 'z' for all mesh types. Now, a CylindricalMesh will have 'r', 'phi', and 'z', and a SphericalMesh will have 'r', 'theta', and 'phi'. This is changed both in the statepoint file and in tallies.out.

Fixes #4017

AI disclosure: Claude Opus 4.8 was used for initial implementation. Generated code was reviewed, revised, and tested by a human (me).

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[tjlaboss]

Upon linting failure (my clang-format was too new), the workflow failed:

Comment with suggestion instructions Unhandled error: HttpError: Resource not accessible by integration

Complete job
Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/github-script@v7. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

https://github.com/openmc-dev/openmc/actions/runs/29595050160/job/87933290028

[GuySten]

Looks good to me.

[paulromano]

I'd like to review this before merging; will try to get to it today or tomorrow.