Fix fixed-mixture density for condensed reactants

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Summary

Fix non-finite density and volume results when Mixture.calc_property is called for liquid or other condensed reactants before equilibrium analysis.

Changes

• Allow fixed-mixture density and volume calculations to include condensed reactant moles in the existing ideal-mixture relation.
• Preserve the default behavior that excludes condensed species from pressure and PV contributions in equilibrium and energy calculations.
• Correct the multi-temperature density and volume unit scaling to match the single-temperature path.
• Add Fortran and Python regression coverage for:
  • Pure O2(L).
  • CH4(L)/O2(L) mixtures.
  • Scalar versus per-species temperatures.
  • Reciprocal density and specific volume.

The CH4(L)/O2(L) reproduction now returns 2.251e-01 g/cc instead of a non-finite value.

Testing

• ctest -R 'cea_core_test' -V
• pytest source/bind/python/tests — 68 passed

Compatibility / Numerical behavior

• No expected changes to numerical results
• Expected changes (explain and provide validation)

Fixed-mixture density and volume results now include condensed reactants and are finite. Multi-temperature density and volume results also receive the same unit scaling as the single-temperature path.