PR summary

Closes #18985

Why is this change necessary?
imshow(..., animated=True) did not behave consistently with other animated artists. Even if animated=True was passed, the returned AxesImage was still registered in Axes.images via add_image, which means it was drawn into the static background on the first render. This is inconsistent with how plot(..., animated=True) works, where animated artists are excluded from the static background and are instead expected to be managed by blitting.

What problem does it solve?
Because animated images were still added to Axes.images, they would appear in the first frame of an animation before the blitting logic ever had a chance to draw them. This breaks the expectation that animated=True means “do not draw this as part of the static background; I will manage it manually for animation.”

What is the reasoning for this implementation?
In Axes.imshow, after creating and configuring the AxesImage, we now branch based on the artist’s animated flag:

if im.get_animated():
    self.add_artist(im)
else:
    self.add_image(im)

• If im.get_animated() is True, we attach the image as a generic Artist using add_artist instead of adding it to Axes.images. This prevents it from being rendered in the initial background draw and matches the behavior of Line2D(animated=True).
• If im.get_animated() is False (the default case), behavior is unchanged: we continue to call add_image(im) and the image is part of the normal background render.

This makes imshow(..., animated=True) consistent with the existing animated artist contract, and fixes the discrepancy discussed in issue #18985.

We also add a new regression test (lib/matplotlib/tests/test_imshow_animated.py) that asserts:

• An image created with animated=True is not present in ax.images, but it is still attached to the Axes (via ax.get_children()), so animation code can still manage/blit it.
• An image created with animated=False is present in ax.images as before.

No interactive drawing (plt.show()) is required in the test.

This should be backwards-compatible for non-animated images. The only behavioral change is for the animated=True case, which now behaves as documented/expected for animated artists.

PR checklist

• "closes Why does setting imshow(animated=True) still show an image? #18985" is in the body of the PR description to link the related issue
• new and changed code is tested (test_imshow_animated.py)
• [N/A] Plotting related features are demonstrated in an example (no new public feature, this is a behavioral fix for an existing keyword)
• [N/A] New Features and API Changes are noted with a directive and release note (this is a bug fix; public API is unchanged)
• [N/A] Documentation changes (no new public API or docstring changes required)