Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

Input:  2 1 3
Output: true

Example 2:

Input:  5 1 4 null null 3 6
Output: false

(Node 4's right child is 4, but 4 < 5, violating BST property at the root)

Constraints:

• The number of nodes in the tree is in the range [1, 10⁴]
• -2³¹ ≤ Node.val ≤ 2³¹ - 1

Input format: BFS level-order, space-separated, null for missing nodes.

Output format: true or false.