fix: #3627619 Saving a node without "administer menu" moves its menu link to first position

Core's menu_ui hides the "Menu settings" section from users who lack "administer menu". Those users never get the tree widget, so no prev_sibling/next_sibling values are submitted. The submit handler read the empty prev_sibling as "place first" and rewrote the weights of the whole branch starting at -50.

Skip repositioning in NodeFormSubmitHandler::handleFormSubmit() when the menu element is not accessible, so the existing parent and weight are kept, as in core. Add a functional test that saves a node as a user without menu access and asserts the link weights are unchanged.

Closes #3627619