gh-50966: Fix unbounded recursion in turtle drag handlers (GH-152626)

gh-50966: Fix unbounded recursion in turtle drag handlers (GH-152626)

TurtleScreenBase._update() redraws with cv.update(), which also reprocesses
input events, so a handler that moves the turtle (such as
screen.ondrag(turtle.goto)) reenters _update() for every queued event until
the interpreter crashes.  A reentrant _update() now only flushes drawing with
update_idletasks().

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>