Fix edge cases with optical setup and runtime

[sethrj]

This adds a high-level coverage test for optical physics with surfaces, originally targeting #1928 , but using a test input that discovered two edge cases we didn't account for:

• Total cross section can be exactly zero in vacuum
• Not all optical materials have to be used in the geometry (we could turn this into a warning instead of a subtle informational message)

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.27%. Comparing base (5336a7f) to head (9ed1cfa).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/celeritas/optical/MaterialParams.cc	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2191      +/-   ##
===========================================
+ Coverage    86.15%   86.27%   +0.11%     
===========================================
  Files         1287     1287              
  Lines        40709    40709              
  Branches     12994    12548     -446     
===========================================
+ Hits         35074    35122      +48     
+ Misses        4624     4414     -210     
- Partials      1011     1173     +162     

Files with missing lines	Coverage Δ
src/celeritas/optical/PhysicsStepUtils.hh	100.00% <100.00%> (ø)
src/celeritas/optical/MaterialParams.cc	90.74% <92.30%> (-0.49%)	⬇️

... and 126 files with indirect coverage changes

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[github-actions]

Test summary

 5 757 files   9 242 suites   17m 50s ⏱️
 2 105 tests  2 076 ✅  29 💤 0 ❌
31 506 runs  31 374 ✅ 132 💤 0 ❌

Results for commit 9ed1cfa.

[amandalund]

Nice, thanks!

[sethrj]

Shit, I didn't update with #2189 before merging.