fix: #3601072 Set fieldset in custom composite

Default the composite wrapper to 'fieldset' when #wrapper_type is not set, so a custom composite left on the default wrapper still renders as a fieldset. It was stripped from the saved element as a default value, and the render code only applied a wrapper when the property was explicitly present, so it fell back to a bare div. Mirrors WebformCompositeFormElementTrait. Updates WebformCompositeCustomTest to assert the wrapper-less composite renders a fieldset.

Closes #3601072