Add symmetry edge-case tests

[galjos]

Summary

• add a real water symmetry regression
• verify both symmetry helpers reject imaginary coordinates
• verify warning suppression does not hide unexpected ComplexWarnings

Validation

• python -m pytest tests/thermo/test_system.py -q
• python -m pytest -q
• python -m pytest --cov=ThermoScreening --cov-report=term-missing -q (94%)
• python -m pylint ThermoScreening

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.54%. Comparing base (90789ff) to head (7faf07a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   93.54%   93.54%           
=======================================
  Files          21       21           
  Lines        1178     1178           
=======================================
  Hits         1102     1102           
  Misses         76       76           

Flag	Coverage Δ
unittests	93.54% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.