Fatal: Default value for props, on update

Problem/Motivation

Following #3549348: Fatal: Default value for props, on load

If the value of a required prop is removed from ContextualFormPanel, no control is done, the prop storage is updated and we have a fatal error (probably Twig\Error\RuntimeError: An exception has been thrown during the rendering of a template The property xxx is required.)

In UI Patterns 2, we rely on Form API blocking ComponentForm submission to avoid this unfortunate situation.

Proposed resolution

Must be done in 2 steps:

• Client side: Don't send the request to display_builder.api_update maybe by adding hx-validate https://htmx.org/attributes/hx-validate/ to onInstanceFormChange and onInstanceUpdateButtonClick HTMX events.
• Server-side:: in ApiController::update(), don't update the instance (InstanceInterface->setSource()) if the form is not validating