[6.x] Resolve dictionary fieldtype permissions via the CP guard

[duncanmcclean]

This pull request fixes an issue where the dictionary fieldtype endpoint resolved the access cp permission using the default auth guard, rather than the guard configured for the Control Panel.

When the default guard differed from the CP guard, dictionary dropdowns in the Control Panel would silently render empty, since the endpoint would return a 403 even for authenticated CP users. Worse, when a non-Statamic user was authenticated on the default guard, the endpoint would return a 500 (Call to a member function isSuper() on null), because the Gate::after callback in the AuthServiceProvider didn't account for User::fromUser() returning null.

This PR fixes it by adding the CP AuthGuard middleware to the dictionary fieldtype route, so the permission check is resolved against the CP guard. Only the guard middleware is applied, not the full CP middleware group, so dictionaries which allow public access remain accessible to guests. This PR also returns null from the Gate::after callback when the authenticated user isn't a Statamic user, so the gate abstains rather than erroring.

Fixes #15204