refactor: Unify upperBound and lowerBound into boundHelper (#7943)

Co-authored-by: Bart <11445373+bthomee@users.noreply.github.com>
Co-authored-by: vlntb <13349202+vlntb@users.noreply.github.com>

include/xrpl/shamap/SHAMap.h

@@ -592,6 +592,20 @@ private:
     SHAMapLeafNode*
     belowHelper(NodePathStack& stack, BelowDirection direction) const;
 
+    /**
+     * Returns the nearest item strictly past `id`, in the given direction.
+     *
+     * Walks back up the path to `id`. At each inner node the branches beyond the one `id` takes
+     * hold the candidates, so the first non-empty one is the closest and the extreme leaf below
+     * it is the answer.
+     *
+     * @param id The key to search from, which need not be in the map.
+     * @param direction First to search upwards from `id`, Last to search downwards.
+     * @return An iterator to the item found, or end() if no item lies on that side of `id`.
+     */
+    ConstIterator
+    boundHelper(uint256 const& id, BelowDirection direction) const;
+
     // Simple descent
     // Get a child of the specified node
     SHAMapTreeNode*