Fix FigureImage losing its transform when re-added via add_artist - #32380

PR summary

Closes #31730.

Figure.figimage() creates a FigureImage without setting a transform. Its offsets are in pixels, so it only drew correctly because an unset transform defaults to identity. After .remove() and fig.add_artist(im), add_artist sees is_transform_set() is False and gives the image transSubfigure, so the image is drawn in the wrong place and doesn't show up.

FigureImage.__init__ now sets IdentityTransform() explicitly, which is what @rcomer suggested in the issue. This covers both figimage() and a FigureImage created directly, and a transform= kwarg still overrides it. The other half of the issue (draw order relative to the axes) was already fixed by #31746, so with both changes the remove/re-add cycle now draws correctly.

AI Disclosure

I used Claude Code (generative AI) to trace the cause through add_artist, draft the regression test, and help write this PR description (to make sure I am following standards).

PR quality check

• Use an expressive title.
• New and changed code is tested
• Plotting related features are demonstrated in an example
• New features and API changes have release notes
• Documentation complies with general and docstring guidelines

N/A for example/release notes/docs: this is a bug fix that makes an existing workflow behave as expected, with no API change.