[BradyPlanden]

Description

#5690's unstructured plotting tests attach a mesh by setting .mesh on a symbol. #5779 made symbols immutable, so on main 27 tests in test_plot_vtk.py and test_unstructured_plot_grid.py fail on every platform:

AttributeError: Cannot set 'mesh' on StateVector 'y[0:1]': symbols are immutable once constructed.

This builds those symbols with with_mesh() instead. It changes only tests. The same commit is in #5785, where CI caught it.

Type of change

Tests only, so no CHANGELOG entry.

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.00%. Comparing base (f4f8de1) to head (b0c670b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5796      +/-   ##
==========================================
- Coverage   98.20%   98.00%   -0.21%     
==========================================
  Files         345      346       +1     
  Lines       34857    35259     +402     
==========================================
+ Hits        34232    34554     +322     
- Misses        625      705      +80     

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