What is the bug?

In #3805, the validate() method of the MLCreateConnectorRequest was used to add field validation using StringUtils.

Unfortunately, validate() is called from the TransportActionNodeProxy class with an org.opensearch.action.Request class, and the StringUtils class is not available to the classloader used for this call.

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Attempt to create a connector using the ML Client's createConnector() method from another plugin (for example, any Flow Framework plugin integ test that creates a connector).
2. Observe the request failing silently with an exception (probably class loading?) swallowed silently somewhere.
3. Add debug logging to see that validate() was working fine but the call to validateFields() was never completing successfully. (Was never able to reproduce an exception on the logging thread.)

What is the expected behavior?

A connector is created or field validation fails with a 400 error.

What is your host/environment?

Easy to reproduce locally on macOS running Flow Framework integ tests on current main branch beginning with the commit including #3805.

Also has caused integ test failures on all platforms on GitHub Action integ tests as well as Jenkins test environment, see opensearch-project/flow-framework#1143

Do you have any additional context?

I've worked around the problem locally by separating the field validation code from the validate() method.

    @Override
    public ActionRequestValidationException validate() {
        // This is called with a Request class and a different classloader so must be simple
        if (mlCreateConnectorInput == null) {
            return addValidationError("ML Connector input can't be null", null);
        }
        return null;
    }

    // This must be called from a thread with access to this class's packages
    public static ActionRequestValidationException validateFields(MLCreateConnectorInput mlCreateConnectorInput) {
        Map<String, FieldDescriptor> fieldsToValidate = new HashMap<>();
        fieldsToValidate.put("Model connector name", new FieldDescriptor(mlCreateConnectorInput.getName(), true));
        fieldsToValidate.put("Model connector description", new FieldDescriptor(mlCreateConnectorInput.getDescription(), false));

        return StringUtils.validateFields(fieldsToValidate);
    }

And then, from TransportCreateConnectorAction:

    @Override
    protected void doExecute(Task task, ActionRequest request, ActionListener<MLCreateConnectorResponse> listener) {
        MLCreateConnectorRequest mlCreateConnectorRequest = MLCreateConnectorRequest.fromActionRequest(request);
        MLCreateConnectorInput mlCreateConnectorInput = mlCreateConnectorRequest.getMlCreateConnectorInput();
        // existing protocol, tenant, and dryrun conditionals
        String connectorName = mlCreateConnectorInput.getName();
        try {
            // add this line
            MLCreateConnectorRequest.validateFields(mlCreateConnectorInput);
            // remainder of the class

Whether to use a static or instance method or validate as part of fromActionRequest() is left as a style preference; the goal here was to identify a workaround, not the chosen solution. It may be useful to put the validation in the MLCreateConnectorInput class rather than here.

The same pattern may be necessary for other Transport Actions updated in #3805 (update connector, register/update model and model group), as validate() will suffer the same shortcoming there.