[Tags] Discussions using nested secondary tags without primary after move

On the recent discuss.flarum move, it was discovered that moving a primary tag X under another primary Y (so that X becomes a nested secondary tag under Y) will not auto-add the new primary Y tag to any discussions under X. This may be unexpected behavior (@tobscure weigh in?) as a user cannot start a new discussion with this setup (using nested X but not Y).

Start by reproducing the reported move from primary tag X under primary tag Y, then inspect how discussions retain their tags after the move. Confirm whether discussions under X lack Y and whether new discussions with nested X require Y. Done means the behavior is defined and covered by a regression test, with the issue's expected outcome resolved.

Written by the indexing model from the issue text.