Fixes #1656: preserve MHD calculations with case optimization

[StevenXue98]

Description

Replace case-optimization guards based on num_dims with guards based on num_vels for three-component MHD calculations in the HLL and LF Riemann solvers.

MFC retains three velocity and magnetic-field components in 1D and 2D MHD cases. The previous dimensional guards compiled out required magnetic-pressure, relativistic-state, and energy-flux calculations in case-optimized lower-dimensional builds, causing the standard 2D MHD rotor example to produce NaNs.

The revised guards continue pruning three-component expressions from optimized lower-dimensional non-MHD builds while retaining them when num_vels=3.

Closes #1656.

Type of change

• Bug fix

Testing

Tested with GNU Fortran 13.3.0 and Open MPI 4.1.6 on CPU with the following case-optimized HLL MHD cases:

• 2D ideal-MHD rotor:

./mfc.sh run examples/2D_mhd_rotor/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

• 1D ideal-MHD Brio–Wu:

./mfc.sh run examples/1D_brio_wu/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

• 1D ideal-MHD Dai–Woodward:

./mfc.sh run examples/1D_dai_woodward/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

• 2D ideal-MHD Orszag–Tang:

./mfc.sh run examples/2D_orszag_tang/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

• Reduced 1D relativistic-MHD case:

./mfc.sh run tests/E0015186/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

• Reduced 2D relativistic-MHD case:

./mfc.sh run tests/33FA33BE/case.py \
  -t pre_process simulation -n 1 -j 1 \
  --no-debug --no-gpu --case-optimization

All cases completed successfully with exit code 0. These runs cover the corrected HLL paths for ideal and relativistic MHD in both 1D and 2D.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

• GPU results match CPU results
• Tested on NVIDIA GPU or AMD GPU

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.53%. Comparing base (bcd64d0) to head (c47915f).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

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  Coverage   59.53%   59.53%           
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  Files          83       83           
  Lines       21119    21119           
  Branches     3132     3132           
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  Hits        12574    12574           
  Misses       6439     6439           
  Partials     2106     2106           

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