Reject chemistry with num_fluids > 1 in case validator

[sbryngelson]

Description

Closes #1470.

Chemistry in MFC assumes a single reacting gas phase and is not coupled to the multi-fluid (5-equation) model. When chemistry = T is combined with num_fluids > 1, the simulation silently produces NaN rather than erroring out. Two paths in src/common/m_variables_conversion.fpp are responsible:

1. The chemistry branch of s_convert_conservative_to_primitive_variables overwrites all partial densities (qK_prim_vf(1:num_fluids)) with the species-summed total density — correct for num_fluids = 1, destructive otherwise.
2. s_compute_pressure recovers temperature from the total internal energy via Cantera's ideal-gas EOS, ignoring pi_inf. With a second stiffened-gas fluid present (e.g. a water droplet, pi_inf ~ 4e8), the inversion diverges and NaN propagates.

Full multi-fluid + chemistry coupling is a larger, research-level effort. This PR is the short-term fix: a case-validator guard that rejects the unsupported combination with a clear message instead of leaving users to debug NaNs. Both chemistry and num_fluids are known at case-definition time, so the guard lives in Python (case_validator.py); no Fortran-side check is needed.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Scope

• Adds one prohibit to CaseValidator.check_chemistry, mirroring the existing mhd and num_fluids != 1 guard.

How Has This Been Tested?

Validator unit-checked directly:

• chemistry='T', num_fluids=2 -> rejected with "chemistry is only supported for single-component flows (num_fluids = 1)"
• chemistry='T', num_fluids=1 -> passes (no regression for existing chemistry examples, all of which use num_fluids = 1)
• num_fluids=2 without chemistry -> passes (unaffected)
• chemistry='T' with num_fluids unset -> passes (deferred to existing model_eqns/num_fluids check)

ruff check and ruff format --check pass on the changed file.

Checklist

• I have added a concise diagnostic message
• My change is a single logical commit
• ruff lint/format pass

[Copilot]

Pull request overview

This PR adds a case-validator guard to reject unsupported chemistry configurations with more than one fluid, preventing known NaN-producing multi-fluid chemistry runs from reaching the Fortran simulation path.

Changes:

• Reads num_fluids in CaseValidator.check_chemistry.
• Prohibits chemistry = T when num_fluids is set to a value other than 1.
• Adds an explanatory comment tying the guard to the single-reacting-gas chemistry assumption.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.31%. Comparing base (574e53d) to head (797eea9).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1472   +/-   ##
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  Coverage   61.31%   61.31%           
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  Files          72       72           
  Lines       19771    19771           
  Branches     2852     2852           
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  Hits        12123    12123           
  Misses       5699     5699           
  Partials     1949     1949           

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