_load_companion_route() {
        const companion = this.app.routes['upgrade'][this.pattern];
        if (companion) {
            // Use existing companion route as it is a user assigned route
            this.#upgrade_with = companion;
        } else {
            // Create and use a temporary default route to allow for healthy upgrade request cycle
            // Default route will upgrade all incoming requests automatically
            this.app._create_route({
                method: 'upgrade',
                pattern: this.pattern,
                handler: (request, response) => response.upgrade(), // By default, upgrade all incoming requests
                options: {
                    _temporary: true, // Flag this route as temporary so it will get overwritten by user specified upgrade route
                },
            });

            // Store created route locally as Server will not call _set_upgrade_route
            // This is because this WebsocketRoute has not been created synchronously yet
            this.#upgrade_with = this.app.routes['upgrade'][this.pattern];
        }
    }
Loads a companion upgrade route from app routes object.
@private