Issue #3605652: Render site settings vertical-tabs panes only once

The element_info_alter detection only matched processGroup/preRenderGroup, so vertical_tabs (which handles groups via processVerticalTabs/ preRenderVerticalTabs) had the generic group callbacks added on top and injected its panes twice. Recognize native vertical-tabs handling, and build default-environment panes as siblings of the vertical_tabs container (standard pattern) with #parents pinned so submitted values are unchanged.

Closes #3605652