phreeqc2026 engine: ensure unit osmotic coefficient; refactor tests

[rkingsbury]

Summary

This PR ensures that the phreeqc2026 engine returns a unit osmotic coefficient, unless PHREEQC itself returns a value other than zero. This is necessary to ensure that osmotic pressure calculations work as intended.

It also contains a bugfix where, if a user tries to use get_osmotic_coefficient without having first run equilibrate, there would be an error because the ppsol attribute has not been initialized.

Finally, unit test files are renamed and a new unit test added for both phreeqc and phreeqc2026 engines.

[rkingsbury]

FYI @vineetbansal ; no action needed but if you see any issues here, let me know.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.42%. Comparing base (e06a242) to head (c623744).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pyEQL/engines.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #333      +/-   ##
==========================================
- Coverage   86.42%   86.42%   -0.01%     
==========================================
  Files          14       14              
  Lines        1849     1856       +7     
  Branches      320      322       +2     
==========================================
+ Hits         1598     1604       +6     
  Misses        207      207              
- Partials       44       45       +1     

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