public static WebSocketCloseStatus ofPrivateUse(int code, String reasonPhrase) {
        checkArgument(code >= 4000 && code < 5000, "code: %s (expected: 4000 <= code < 5000)", code);
        checkArgument(reasonPhrase.getBytes().length <= 125, "reasonPhrase: %s (expected: <= 125 bytes)",
                      reasonPhrase);
        return new WebSocketCloseStatus(code, requireNonNull(reasonPhrase, "reasonPhrase"));
    }
Creates a new {@link WebSocketCloseStatus} whose status is in the range 4000-4999 for private use.
See <a href="https://datatracker.ietf.org/doc/html/rfc6455#section-7.4.2">rfc6455</a>.

<p>Note that {@code reasonPhrase} must be 125 bytes or less.
core/src/main/java/com/linecorp/armeria/common/websocket/WebSocketCloseStatus.java