Given the root of a binary tree, determine if it is a valid binary search tree (BST).
A valid BST is defined as: every node in a node's left subtree has a value strictly less than the node's value, every node in its right subtree has a value strictly greater, and both subtrees must also be valid BSTs. Duplicate values are not allowed.
Trees in this problem are given as a level-order array: null marks a missing child, and trailing nulls are omitted.
Example:
Input: root = [2,1,3]
Output: true