Validate BST

Implement a function to check if a binary tree is a binary search tree.

BST Property: For every node, all values in the left subtree are strictly less than the node, and all values in the right subtree are strictly greater.

Examples:

       5
      / \
     3   7     →  True (valid BST)
    / \
   2   4

       5
      / \
     3   7     →  False (6 is in left subtree of 7 but > 5)
    / \
   2   6