Issue enabling GraphQL subscriptions in Go; Playground doesn't detect the subscription - 'schema does not support subscriptions.'

Question

In a GraphQL API in Go, I implemented subscriptions using gqlgen. I defined the 'Subscription' type to return either a boolean or the current server time. I ran the 'generate' command to apply the changes and obtain the autogenerated code. I implemented the logic in 'schemaresolvers.go' using goroutines. In 'main.go,' I configured the server with transports and websockets. However, the Playground does not detect the subscriptions, and I receive the following error when attempting to use 'subscription' in the Playground:

'Schema does not support operation type "subscription".'

"github.com/99designs/gqlgen/graphql/handler/transport"

main.go

func NewGraphQLHandler(dbInstance *gorm.DB) (http.Handler, error) {
    c := generated.Config{
        Resolvers: &graph.Resolver{
            DB: dbInstance,
        },
    }
    c.Directives.Auth = directives.Auth
    srv := handler.NewDefaultServer(generated.NewExecutableSchema(c))
    //agregar websocket, no detecta la subscription en el playground
    srv.AddTransport(&transport.Websocket)

    return srv, nil
}

Configuration main.go

func registerHandlers(lifecycle fx.Lifecycle, router *mux.Router, graphqlHandler http.Handler) {
    router.Handle("/", playground.Handler("GraphQL playground", "/query"))
    router.Handle("/query", graphqlHandler)
    router.Handle("/subscriptions", graphqlHandler)

schemaresolvers.go

func (r *subscriptionResolver) ServerEvents(ctx context.Context) (<-chan bool, error) {

    //creamos el channel
    result := make(chan bool)

    //go routine
    go func() {

        //el ping cada 5 segundos que se enviara al canal
        ticker := time.NewTicker(5 * time.Second)
        defer ticker.Stop()

        for {
            select {
            case <-ticker.C:
                result <- true //se envia true al canal
            case <-ctx.Done():
                //cuando el cliente se desconecta se cierra
                close(result)
                fmt.Println("Subscripcion cerrada")
                return
            }
        }
    }()
    return result, nil
}

Schema

type Subscription {
    serverEvents: Boolean!
}

Moreover, I tried using the suggested solution, but unfortunately, it didn't yield the expected results

 srv := handler.New(generated.NewExecutableSchema(c))

    
    srv.AddTransport(websocket.Transport{
        InitFunc: func(ctx context.Context, payload websocket.InitPayload) (context.Context, error) {
            return ctx, nil
        },
        Upgrader: websocket.Upgrader{
            CheckOrigin: func(r *http.Request) bool {
                return true
            },
        },
    })