Fix move_child noop when target is child.

This finishes the work done in #2530, which was incomplete as seen in #1743 (comment).

src/textual/widget.py

@@ -941,10 +941,6 @@ def move_child(
         elif before is not None and after is not None:
             raise WidgetError("Only one of `before` or `after` can be handled.")
 
-        # We short-circuit the no-op, otherwise it will error later down the road.
-        if child is before or child is after:
-            return
-
         def _to_widget(child: int | Widget, called: str) -> Widget:
             """Ensure a given child reference is a Widget."""
             if isinstance(child, int):
@@ -969,6 +965,9 @@ def _to_widget(child: int | Widget, called: str) -> Widget:
             cast("int | Widget", before if after is None else after), "move towards"
         )
 
+        if child is target:
+            return  # Nothing to be done.
+
         # At this point we should know what we're moving, and it should be a
         # child; where we're moving it to, which should be within the child
         # list; and how we're supposed to move it. All that's left is doing