Add JWT support for internal Subscriptions API

We want to be able to authenticate with a JWT from CustomersDot when
accessing this endpoint. Migrate it to the internal API definition which
has JWT auth, and update the specs to account for this.

MR: !161552
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/466368

ee/lib/api/gitlab_subscriptions/subscriptions.rb

@@ -7,9 +7,9 @@ class Subscriptions < ::API::Base
       urgency :low
 
       before do
-        authenticated_as_admin!
+        @namespace = find_namespace(params[:id])
 
-        @namespace = find_namespace!(params[:id])
+        not_found!('Namespace') unless @namespace
       end
 
       resource :namespaces, requirements: ::API::API::NAMESPACE_OR_PROJECT_REQUIREMENTS do

ee/lib/ee/api/api.rb

@@ -74,7 +74,6 @@ module API
         mount ::API::CodeSuggestions
         mount ::API::Chat
         mount ::API::DuoCodeReview
-        mount ::API::GitlabSubscriptions::Subscriptions
 
         mount ::API::Internal::AppSec::Dast::SiteValidations
         mount ::API::Internal::Search::Zoekt

ee/lib/gitlab_subscriptions/api/internal/api.rb

@@ -24,6 +24,7 @@ class API < ::API::Base
         # 'internal/gitlab_subscriptions', in https://gitlab.com/gitlab-org/gitlab/-/issues/463741.
         # They will then be removed in https://gitlab.com/gitlab-org/gitlab/-/issues/473625.
         mount ::API::GitlabSubscriptions::AddOnPurchases
+        mount ::API::GitlabSubscriptions::Subscriptions
 
         # this API is deprecated, it will be removed in https://gitlab.com/gitlab-org/gitlab/-/issues/473625.
         # Please use GitlabSubscriptions::API::Internal::UpcomingReconciliations instead.