Restore image position after an error in ImageSequence.all_frames()

[niukanen1]

Changes proposed in this pull request:

• Run the existing frame-position restore in ImageSequence.all_frames() when frame collection raises, as well as when it succeeds.
• Add a regression with a custom image that raises during seek(), checking that the exception propagates and the original position is restored.

Previously, an exception while collecting frames left the caller's image at the failed frame. The finally block covers frame collection only; callback execution and successful results keep their existing behavior.

The change is also documented in the 13.0.0 release notes.

Validation on the current head:

• python3 selftest.py — 59 passed.
• pytest -q Tests/test_imagesequence.py — 9 passed.
• Applicable pre-commit hooks for the changed test and release note — passed.

Before the test simplification, the unchanged production fix also passed the full local test suite, repository lint, and mypy.

[radarhere]

Hi. I'm curious - is this a theoretical concern, or something that actually caused a problem for you? If nothing else, I want to make sure that individual Pillow formats aren't raising errors during seek() on valid images.

[radarhere]

I've created niukanen1#1 with suggestions.

[niukanen1]

Thanks for asking. This came from inspecting ImageSequence.all_frames(), not from a user report. I reproduced the state leak with Tests/images/multipage.tiff by starting on frame 2 and deliberately raising a pre-created RuntimeError from Image.Image.copy() on frame 1. Before the fix, the image remained on frame 1. I have not observed any Pillow format raising from seek() on a valid image, so I do not have evidence of a spontaneous format-specific seek failure.

Thanks as well for the fork PR. I applied both suggested commits. The custom Image test exercises the same finally cleanup path more simply, and the release-note wording is more accurate for list input because processing stops at the exception and later images are not visited. The current head passes selftest.py, Tests/test_imagesequence.py, and the applicable pre-commit hooks.