Visualization tests with image regression testing

Describe the new feature or enhancement

PyVista has contract testing with MNE to make sure we don't break anything you all are using. I'm wondering if we want to take this a step further with image regression testing to validate further we aren't changing the visualizations you all create here

AFAIK, that isn't implemented here, but do let me know if I'm not correct.

Describe your proposed implementation

Now that pytest-pyvista is implemented, this should be straightforward to set up for the visualization tests. Though... I looked at the tests here, and it wasn't immediately obvious to me where/how PyVista is used in the tests

https://github.com/pyvista/pytest-pyvista

Describe possible alternatives

Or just leave it as is, which only validates API usage and not actual rendering results.

Additional context

No response

Start by locating the visualization tests mentioned in the issue and reviewing how pytest-pyvista provides image regression testing. Determine where PyVista is used, then define coverage and expected rendered-image comparisons; the work is done when visualization tests validate rendering results rather than API usage alone.