Accessibility: dragover fires immediately after dragstart in sortable

When dragging an item in a sortable, the initial announcements after starting a drag are:

from dragstart: `Picked up draggable item {id}.`
from dragover: `Draggable item {id} was moved over droppable target {id}.` (i.e., itself)

In general, it makes sense for `dragover` fire consistently, but for accessibility, I can't think of a scenario where users would need the second message. If `dragstart` is defined, I'd expect the first `dragover` event after `dragstart` to be suppressed if the source and target id are the same.

Start by locating the sortable accessibility announcement handling for dragstart and dragover, then reproduce the initial self-target announcement. Done means the first dragover after dragstart is suppressed when source and target IDs match, while normal dragover announcements remain unchanged; run the relevant test suite to verify the behavior.

Written by the indexing model from the issue text.