Assessment item/exercise related fixes and tests

[rtibbles]

Summary

• Adds new tests for Perseus archive file generation
• Adds new tests for image resizing
• Adds regression test for mistake that has been in the codebase for 4+ years courtesy of me, where we were checking for the incorrect placeholder to discover embedded files in assessment items
• Updates assessment item endpoint to look for the correct place holder - the one that is actually used in the frontend
• Updates assessment item endpoint and tests for cases where file object does not exist, but file does exist, and case where neither exist

References

This was mostly motivated by having some solid regression tests in place in preparation for #4878

Reviewer guidance

Do the perseus file generation tests provide appropriate coverage, and correctly test the behaviour? Note I did use Claude (as noted in code comments) to generate these tests, but I also carefully reviewed them, and made a lot of updates to the non-image-resize tests.

Does the placeholder being used now match up to the one we use in the frontend? https://github.com/learningequality/studio/blob/unstable/contentcuration/contentcuration/frontend/shared/views/MarkdownEditor/constants.js#L12 <-- this is the one used in the frontend, and also by ricecooker: https://github.com/learningequality/ricecooker/blob/develop/ricecooker/classes/questions.py#L226

[rtibbles]

This needs an update to fix linting, and also add a test case for a file existing in cloud storage, but not having an appropriately queryable file object (i.e. their orphaned file object got cleaned up by garbage collection).

[bjester]

I see one instance of IMG_PLACEHOLDER in contentcuration/contentcuration/tests/test_rest_framework.py

Should it be there?

[rtibbles]

I see one instance of IMG_PLACEHOLDER in contentcuration/contentcuration/tests/test_rest_framework.py

Should it be there?

Looks like the whole test case was being skipped (by me, 5 years ago) so I've just deleted it.

[bjester]

LGTM