wp post term set --by=id with a non-existent ID removes all terms and reports success #690

wp post term set <id> <taxonomy> <term-id> --by=id removes every term from the object when none of the given IDs exist, and still prints Success:

$ wp post term add 1 post_tag apple banana
Success: Added terms.
$ wp post term set 1 post_tag 9999 --by=id
Success: Set term.
$ wp post term list 1 post_tag --format=count
0

CommandWithTerms::prepare_terms() drops IDs that do not match a term. When none match, set() calls wp_set_object_terms( $id, [], $taxonomy, false ), which replaces the existing terms with nothing. wp post term add --by=id takes the same path: it adds nothing and prints Success: Added term.. wp user term set|add use the same class.

Expected: an error, and the existing terms stay as they are.