Function collect_boundary_types 

pub fn collect_boundary_types(
    types: &HashMap<String, TypeDecl>,
    services: &HashMap<String, ServiceDecl>,
    agents: &HashMap<String, AgentDecl>,
) -> Vec<String>

Compute the set of type names (transitively reachable) that need serialise/deserialise helpers for this context: any type used in the argument or return position of a service handler exposed by this context, walked through record fields, sum payloads, and the generic type parameters of Result/Option/Effect.