Function not_found_json 

pub fn not_found_json(ids: &[Id]) -> Value

Convert a slice of Ids to a JSON notFound value.

RFC 8620 §5.1 specifies notFound as Id[] — always an array, never null. Returns an empty array when all requested ids were found.

Equivalent to serde_json::to_value(ids) but threads through Value::Array directly so the call site is infallible (bd:JMAP-wlip.28).