fix: Handle Taxonomy CASCADE deletes in _is_explicit_tag_delete

[kdmccormick]

Merge info

For Verawood, we need to merge either this PR, or just the straightforward revert: #577. Both are ready to go.

EDIT: #576 is also a potential fix.

Description

When a Taxonomy is deleted, Django fires pre_delete for each related Tag with origin set to the Taxonomy instance. The previous code raised a TypeError for any non-Tag, non-QuerySet origin, breaking any consumer that deletes a Taxonomy.

For a non-Tag-queryset origin (e.g. taxonomy.delete() or Taxonomy.objects.filter(...).delete()), we now emit only for root-level tags; their handler covers the whole subtree via lineage__startswith.

Bumps to v1.0.1.

Supporting information

The original code was added in #571

while upgrading to v1.0, openedx-platform unit tests caught this:
https://gist.github.com/kdmccormick/912e71d4a57bf8aed046e9be3ca297e7

AI

Generated by Claude Sonnet 4.6 with my prompt:

(We're working within the src/openedx_tagging directory, no need to look at other subdirs of src)
In 508be7d, test_is_explicit_tag_delete_raises_for_unexpected_origin_type checks that a Taxonomy cannot be passed as the origin to _is_explicit_tag_delete. But this same behavior is causing failures in external consumes of this package, because deleting a Taxonomy causes a CASCADE which delete tags, thus passing Taxonomy as the origin to _is_explicit_tag_delete. This seems like a bug in openedx_tagging, no? Can you discern why the commit author made the choice to disallow this scenario?

which it got right on the first try. I just had a couple follow-ups to tweak its coding style.

I then had Opus 4.7 audit the change, and it suggested the additional test_taxonomy_delete_updates_search_index test.

I reviewed all code myself.

[ormsbee]

This seems fine to me, and I would prefer to fix forward.

Need to check my assumptions about how taxonomy deletes are already handled.