Add tree validator

[JoshdRod]

Missing input validation. The function doesn't validate if the tree structure is valid before attempting to render it to MathJax. If the tree has invalid node references (e.g., leftNode or rightNode pointing to non-existent indices), this will cause runtime errors. Add validation to check that node indices are within bounds before accessing them.

Originally posted by @Copilot in #8 (comment)