Fix finding child content-type elements in stage-builder

[orukusaki]

Description (*)

Fixes a bug whereby child content-type elements will not be found, where other types of elements also exist at the same level.

Manual testing scenarios (*)

1. Create a content type, where child content elements are mixed in with other elements. For example:
   master.html:

<div data-bind="attr: data.main.attributes, style: data.main.style">
    <div class="swiper">
        <ul class="swiper-wrapper" each="contentType.getChildren()">
            <li class="swiper-slide">
                <render args="{ name: content.template, data: content }"></render>
            </li>
        </ul>
        <div class="swiper-scrollbar"></div>
    </div>
</div>

preview.html:

<div data-bind="attr: data.main.attributes, style: data.main.style"
     event="{ mouseover: onMouseOver, mouseout: onMouseOut }, mouseoverBubble: false">
    <render args="getOptions().template"></render>
    <div class="swiper">
        <ul class="swiper-wrapper element-children content-type-container"
            each="contentType.getChildren()" css="getChildrenCss()" attr="{id: contentType.id + '-children'}"
            data-bind="sortable: getSortableOptions()"
            afterRender="function (element) { if (typeof afterChildrenRender === 'function') { afterChildrenRender(element); } }">
            <li class="pagebuilder-content-type-wrapper swiper-slide">
                <if args="$parent.isContainer()">
                    <div class="pagebuilder-drop-indicator"></div>
                </if>
                <div
                    template="{ name: preview.template, data: preview, afterRender: function () { preview.dispatchAfterRenderEvent.apply(preview, arguments); } }"
                    attr="{ id: id }" css="{'pagebuilder-content-type-hidden': !preview.display()}">
                </div>
                <if args="$parent.isContainer() && $index() === $parent.contentType.getChildren()().length - 1">
                    <div class="pagebuilder-drop-indicator"></div>
                </if>
            </li>
        </ul>
        <div class="swiper-scrollbar"></div>
    </div>
</div>

It is possible to create this content type in Pagebuilder, and the correct result is rendered into the database and on the frontend. However, Pagebuilder cannot then load up the content back into the stage - the presence of the <li> element causes only the last item to be found, and <div class="swiper-scrollbar"> causes even that to be forgotten.

This small fix corrects this, and all types of content can now be loaded into the stage correctly.

Checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)