Add analytic tests for ray-traced intersection distances

[mazy1998]

Description

Add focused C++ unit tests for Ray::trace() using a concentric-sphere CSG geometry.

The tests verify:

• the analytic chord length through a sphere, 2 * sqrt(R^2 - b^2);
• all four cumulative intersection distances for a ray crossing a spherical shell and its inner region.

Together, these cases exercise entry from outside the model, multiple internal surface crossings, re-entry into a cell, and final leakage through a vacuum boundary. This adds direct analytic coverage for the reusable Ray implementation generalized in #3816 and separated into its own source files in #3845.

No production behavior or public API is changed.

Testing

• cmake --build build --target test_ray -j2
• ctest --test-dir build -R '^test_ray$' --output-on-failure
• All nine locally available C++ test executables pass; the optional MCPL test was excluded because MCPL is not installed.
• A local mutation check confirmed the assertions detect TINY_BIT-scale errors in accumulated traversal distances.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on the C++ source file
• I have followed the style guidelines for Python source files (not applicable)
• I have made corresponding changes to the documentation (not applicable; test-only change)
• I have added tests that prove the covered behavior works