Require two fluids for phase change

[rj-scharff]

Description

Reject phase-change configurations with fewer than two carrier fluids.

The phase-change operator uses fixed liquid and vapor indices lp = 1 and
vp = 2 and writes both indexed fields. With one carrier fluid, the vapor
index aliases x-momentum.

This implements the narrow validation guard requested in Issue #1738. It
intentionally does not address the broader phase-change/Euler–Euler
compatibility question.

Fixes #1738.

Type of change

• Bug fix

Testing

• Added direct validator coverage for:
  • disabled phase change;
  • missing num_fluids;
  • one fluid;
  • two fluids;
  • three fluids.
• TestPhaseChangeFluidPairing: 5 passed.
• Complete test_case_validator.py: 51 passed.
• Complete ./mfc.sh lint: passed.
  • Ruff passed for toolchain, examples, and benchmarks.
  • 459 bundled tests passed with 8 warnings and 9 subtests passed.
• Validator-only one-fluid probe: the new constraint was reported for all three
  validation stages.
• Validator-only two-fluid phase-change probe: passed all three stages.
• Unmodified 2D_phasechange_bubble positive control: passed all three stages.
• Normal pre-commit hook: all seven stages passed.
• git diff --check: passed.

No preprocessing or simulation of the unsafe one-fluid configuration was
performed.

Checklist

• I added or updated tests for the new behavior.
• The change is limited to the validator and focused validator tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.59%. Comparing base (d4ffa29) to head (0e8f16c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1743   +/-   ##
=======================================
  Coverage   61.59%   61.59%           
=======================================
  Files          84       84           
  Lines       21493    21493           
  Branches     3176     3176           
=======================================
  Hits        13239    13239           
  Misses       6078     6078           
  Partials     2176     2176           

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