[Bug]: TagParents not being deleted when deleting Tag

[CyanVoxel]

Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

TagStudio Version

main branch (Alpha 9.5.4+)

Operating System & Version

macOS 15.6.1

Description

TagParents are not being deleted from the tag_parents table in the database when deleting a Tag from the tags table. I came across this error before #1103 was merged but didn't track down the cause until afterwards.

Expected Behavior

When a tag is deleted from the database, all references of that tag should also be removed. This includes any rows in tag_parents in which that tag's ID was involved in a relationship.

Any current libraries that are affected by this issue also need to be repaired and have these invalid tag_parents deleted.

Steps to Reproduce

1. Create tag, example "Parent"
2. Create tag, example "Child"
3. Add Parent tag to Child's parent tags
4. Add Parent tag to an entry
5. In the tag manager, delete the "Parent" tag
6. Click on the entry to view the traceback in the console, or investigate the DB to see that the tag_parent row is still present

Logs

When viewing an entry that has a tag referencing a deleted parent:

Traceback (most recent call last):
  File "/Users/cyanvoxel/Local/Dev/TagStudio/src/tagstudio/qt/views/preview_panel_view.py", line 165, in set_selection
    self._fields.update_from_entry(entry_id)
  File "/Users/cyanvoxel/Local/Dev/TagStudio/src/tagstudio/qt/mixed/field_containers.py", line 116, in update_from_entry
    self.update_granular(entry.tags, entry.fields, update_badges)
  File "/Users/cyanvoxel/Local/Dev/TagStudio/src/tagstudio/qt/mixed/field_containers.py", line 126, in update_granular
    categories = self.get_tag_categories(entry_tags)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cyanvoxel/Local/Dev/TagStudio/src/tagstudio/qt/mixed/field_containers.py", line 173, in get_tag_categories
    hierarchy_tags = self.lib.get_tag_hierarchy(t.id for t in tags)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cyanvoxel/Local/Dev/TagStudio/src/tagstudio/core/library/alchemy/library.py", line 1608, in get_tag_hierarchy
    tag.parent_tags = {all_tags[p] for p in all_tag_parents.get(tag.id, [])}
                       ~~~~~~~~^^^
KeyError: 1000

[TrigamDev]

I'm still getting this bug (or at least one very similar to it) in Alpha v9.5.5
Specifically, if I have two tags: Tag A and Tag B, with Tag A set to be Tag B's parent, and then delete Tag B, the relationship in the tag_parents table persists. If I instead delete Tag A, though, the row is properly deleted.