fix(ui5-list): announce grouped list item counts correctly - #14228

A ui5-list containing a single ui5-li-group was announced as a "list with 1 item" because the group header was exposed as the list's only listitem, and the real items received inflated positions.

The group header is now a heading (role="heading" with aria-level) rather than a listitem, so it is announced but not counted among the list's items. Each group is wrapped in a labelled role="group", and under the default List role its items are placed in an inner role="list" so they are counted within the group rather than by the outer list.

As a result:

• A single-group list is announced with the real item count.
• The group header is read as a heading (not a counted item).
• The outer list no longer adds a "contains N sub groups" summary.

When the list's accessible-role is ListBox, the group instead keeps its items as direct options of the role="group" (no intervening role="list", header exposed as role="none"), so they stay part of the listbox.

The group's role="group" and inner role="list" are labelled with headerAccessibleName when set, falling back to the visible headerText.