PR Summary

These happen to work because 3D artists are derived from 2D artists in ways that work with the existing legend handlers. But if we ever change that to make 3D better, it might break legends. Thus add some tests to ensure that legends for 3D artists don't break.

This is waiting for the refactor in #24247.

PR Checklist

Tests and Styling

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [n/a] New features are documented, with examples if plot related.
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).