Function validate_tree 

pub fn validate_tree(node: &SyntaxNode<Lang>) -> Result<(), String>

Validates that the CST follows expected structural invariants.

This is useful for testing that mutations don’t break tree structure. Returns Ok(()) if the tree is valid, or an error message if issues are found.

Current checks:

• Every MAPPING_ENTRY has exactly one KEY
• Every MAPPING_ENTRY has exactly one COLON
• Every MAPPING_ENTRY has at most one VALUE
• Block-style MAPPING_ENTRY and SEQUENCE_ENTRY nodes end with NEWLINE

Example

use yaml_edit::{YamlFile, debug};
use rowan::ast::AstNode;
use std::str::FromStr;

let yaml = YamlFile::from_str("name: Alice\n").unwrap();
debug::validate_tree(yaml.syntax()).expect("Tree should be valid");