Add force parameter to deleteAllTerms for proper deletion.

[Marianne380]

What

The deleteAllTerms() function was not properly deleting terms between test runs, causing test pollution and flaky tests.

Why

The WordPress REST API requires the force=true parameter when deleting terms via the API. Without it, the API returns a 501 error:

"Terms do not support trashing. Set force=true to delete.", see https://github.com/WordPress/wordpress-develop/blob/6.9.0/src/wp-includes/rest-api/endpoints/class-wp-rest-terms-controller.php#L783.

Fix

Add ?force=true to the DELETE request path in deleteAllTerms():