amcheck: Allow interrupting the child-level rightlink walk

bt_child_highkey_check() walks right along the child level following
btpo_next links, reading a page on each iteration, but its loop lacked a
CHECK_FOR_INTERRUPTS().  Every other page-traversal loop in
verify_nbtree.c already has one.

The loop's existing checks do not guarantee a timely exit on a corrupt
index.  The in-loop cycle check only fires when a rightlink points back
through the block the walk started from, or to a page whose btpo_prev
points to itself, so a cycle further downstream can go undetected -- for
example a run of pages all flagged P_INCOMPLETE_SPLIT, which skips both
the high-key comparison and bt_downlink_missing_check() and so never
reaches an error.  Such an index, or simply a very long rightlink chain,
makes the walk effectively uninterruptible.  Since amcheck exists to be
run against possibly-corrupt indexes, make this walk respond to query
cancellation and shutdown requests like the sibling loops do.

Author: Paul Kim <mok03127@gmail.com>
Reviewed-by: shihao zhong <zhong950419@gmail.com>
Discussion: https://www.postgresql.org/message-id/20260717151437.10713-1-mok03127@gmail.com

contrib/amcheck/verify_nbtree.c

@@ -2191,6 +2191,8 @@ bt_child_highkey_check(BtreeCheckState *state,
 	/* Move to the right on the child level */
 	while (true)
 	{
+		CHECK_FOR_INTERRUPTS();
+
 		/*
 		 * Did we traverse the whole tree level and this is check for pages to
 		 * the right of rightmost downlink?