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: root = [2,1,3]
Output: true
```
**Example 2:**
```
Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's right child is 4, which is less than 5.
```