What is the problem?
The tag groups search endpoint (`TagGroupsController#search`) returns
tag data with only `id` and `name` fields. The `slug` field is missing
from the response, which can cause issues on the client side when it
needs the slug to build URLs or match tags correctly — especially for
tags whose slug differs from their name.
What is the solution?
Add `slug` to the `pluck` and `map` calls in
`TagGroupsController#search` so the response includes the tag slug
alongside `id` and `name`.