Function extract_handler_signature 

pub fn extract_handler_signature(func: &ItemFn) -> Result<HandlerSignature>

Extract a HandlerSignature from a handler function.

Validates:

• The return type is Json<T> or Result<Json<T>, E> (not a bare type)
• Collects State<S>, Json<T>, and Query<T> parameters when present

Handlers that return neither Json<T> nor Result<Json<T>, E> are rejected with an error pointing at the return type token.