Hotfix for Total Volume bug

[RemDelaporteMathurin]

Description

Summary

For Discontinuous cases, the export_vol variable was defined in the wrong loop, leading to wrong results if surface fluxes were defined first or even errors if only TotalVolume quantities were defined.

Related Issues

Motivation and Context

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🔨 Code refactoring (no functional changes, no API changes)
• 📝 Documentation update
• ✅ Test update (adding missing tests or correcting existing tests)
• 🔧 Build/CI configuration change

Testing

• All existing tests pass locally (pytest)
• I have added new tests that prove my fix is effective or that my feature works

Code Quality Checklist

• My code follows the code style of this project (Ruff formatted: ruff format .)
• My code passes linting checks (ruff check .)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

Documentation

• I have updated the documentation accordingly (if applicable)
• I have added docstrings to new functions/classes following the project conventions

Breaking Changes

Screenshots/Examples

Additional Notes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.77%. Comparing base (b09927e) to head (2b3a99f).
⚠️ Report is 2 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx    #1083   +/-   ##
========================================
  Coverage    94.77%   94.77%           
========================================
  Files           44       44           
  Lines         3175     3175           
========================================
  Hits          3009     3009           
  Misses         166      166           

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