bug: make sure symbol always has required mesh attribute

[martinjrobins]

Description

Fixes #4637

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (72c23ea) to head (ea15ec9).
Report is 158 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4644   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          302      302           
  Lines        22858    22866    +8     
========================================
+ Hits         22679    22687    +8     
  Misses         179      179           

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