Fixes #187543
Fixes #143156

_DragAvatar.updateDrag bails early when the hit-tested targets share a
prefix with _enteredTargets. That optimization (the pass-through path,
refined in #60174) is only correct once a target has accepted the drag
(_activeTarget != null), because deeper targets below the active one are
intentionally ignored. When nothing has accepted yet, _enteredTargets
contains every hit target, so a newly-appearing lower target makes the hit
list longer while still prefix-matching, and the code bailed without
entering it. Dragging over an upper rejecting target and then into an
overlap therefore left the lower target unrecognized.

The fix only takes the early-bail when _activeTarget != null or the target
set is exactly unchanged, so the fast-path is preserved once a target has
accepted, while a newly-appearing lower target is re-evaluated when nothing
has accepted.

Tests

Added a regression test (over an upper rejecting target, then into the
overlap, the lower target now accepts; verified it fails without the fix and
passes with it) and a guard test (when the upper target accepts, the drag
does not fall through to a lower overlapping target). The full
draggable_test.dart suite passes.

Design note

When nothing has accepted yet and a new lower target appears, the existing
leave/re-enter path re-evaluates the targets, which fires onLeave once on
the still-hovered upper target before re-entering it. This matches existing
behavior whenever the entered-target set changes and is intentionally
minimal; a plain == check would instead defeat the accepted-target
fast-path.