Function expression_to_debug_tree 

pub fn expression_to_debug_tree(
    expr: &Expression,
    context: &TypeContext,
) -> Value

Convert a FHIRPath expression AST to a JSON debug tree

The output format matches the structure expected by fhirpath-lab:

{
  "ExpressionType": "BinaryExpression",
  "Name": "|",
  "Arguments": [...],
  "ReturnType": "string[]"
}