Closes #31319

## PR summary
This PR fixes a crash reported in #31319 that occurs when removing a subfigure after creating subplots inside it.

### Why is this change necessary?
Calling `.remove()` on a `SubFigure` can currently raise:
```pytb
AttributeError: 'list' object has no attribute '_mouseover_set'

This makes subfigure removal unreliable in otherwise valid figure layouts.

What problem does it solve?

It prevents Artist.remove() from attempting axes-specific mouse-hitlist cleanup when self.axes is not an actual matplotlib.axes.Axes instance (it can be a list in the subfigure-removal scenario).

What is the reasoning for this implementation?

Artist.remove() contains cleanup code that assumes self.axes is an Axes and accesses self.axes._mouseover_set. In the failing case, self.axes is a list, so accessing _mouseover_set crashes. The fix tightens the guard so this cleanup block only runs when self.axes is a non-None Axes instance.

Minimal self-contained example (repro / verification)

import matplotlib
matplotlib.use("Agg")  # for reproducibility/headless runs
import matplotlib.pyplot as plt

fig = plt.figure()
subfigs = fig.subfigures(1, 2)
subfigs[0].subplots()
subfigs[0].remove()
print("OK: no crash")

What changed (only 2 files)

• lib/matplotlib/artist.py: Guard the _mouseover_set cleanup in Artist.remove() so it only runs when self.axes is an Axes.
• lib/matplotlib/tests/test_figure.py: Add/update a regression test that creates subplots inside a subfigure and then removes the subfigure.

How to test locally

python -m pytest lib/matplotlib/tests/test_figure.py -k test_subfigure_remove -q

AI Disclosure

I used AI assistance to help draft the PR description and verification steps. The code changes and local verification were performed and reviewed by me.

PR checklist

• "closes [Bug]: Crash when removing a subfigure with a subplot in a figure #31319" is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines