Orphaned categories can be created by accident

[dzver]

There are two ways to do that.

1. You can work on category tree from two browser windows.

• In window one, delete a category
• In window two, add a child to the category that has been deleted.

wp_insert_term needs to check if the parent exists.

2. If you delete a couple of categories with bulk-delete, a third level child node can become orphan.

wp_delete_term uses get_term to find the parent node and can retrieve cached data.

[solarissmoke]

Related: #8119

[dlh]

Diff attempts to address issue (1). After the $args are parsed, checks term_exists() if needed.

[wonderboymusic]

We should inspect this with unit tests

[wonderboymusic]

In 29196:

Avoid a race condition when multiple windows are open so that orphaned terms cannot be created by accident.

Adds a unit test.

Props dlh.
Fixes #19205.

[SergeyBiryukov]

In 29234:

Use an existing string.

see #19205.