Fix broken master; ensure all product service APIs have correct implementations (#20663)

* fix broken master; ensure all product service APIs are implemented by the structs passed to products

* use zero allocation interface checks

app/server.go

@@ -55,6 +55,7 @@ import (
 	"github.com/mattermost/mattermost-server/v6/jobs/resend_invitation_email"
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/plugin/scheduler"
+	"github.com/mattermost/mattermost-server/v6/product"
 	"github.com/mattermost/mattermost-server/v6/services/awsmeter"
 	"github.com/mattermost/mattermost-server/v6/services/cache"
 	"github.com/mattermost/mattermost-server/v6/services/httpservice"
@@ -394,6 +395,9 @@ func NewServer(options ...Option) (*Server, error) {
 		return nil, errors.Wrapf(err, "unable to create teams service")
 	}
 
+	// ensure app implements `product.UserService`
+	var _ product.UserService = (*App)(nil)
+
 	serviceMap := map[ServiceKey]any{
 		ChannelKey:       &channelsWrapper{srv: s},
 		ConfigKey:        s.configStore,