When a `term_id` matches in `_get_term_children()`, recurse through its...

When a `term_id` matches in `_get_term_children()`, recurse through its children until there is no more depth in the hierarchy. Since `get_terms()` return terms with a `count` of `0` when their children are not empty, we must return all children so that `get_terms()` can check their count.

In [27108], #26903 was fixed, but only because we were using the example in the ticket, leaving out infinite depth for hierarchical taxonomies.

Adds unit tests, including `Tests_Term_getTerms::test_get_terms_seven_levels_deep()`.

Fixes #26903. Again.


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