@VisibleForTesting
  public String url() {
    if (httpServer == null) {
      return "";
    }
    String scheme = config.isTlsEnabled() ? "https" : "http";
    return NetworkUtility.urlForSocketAddress(scheme, socketAddress());
  }
Returns the URL of the GraphQL HTTP service.

<p>This method constructs and returns the URL that the HTTP server is bound to. If the server
is not running, it returns an empty string.

@return the URL of the HTTP server, or an empty string if the server is not running.
ethereum/api/src/main/java/org/hyperledger/besu/ethereum/api/graphql/GraphQLHttpService.java