Function handle_get 

pub async fn handle_get<O: GetObject, B: JmapBackend>(
    backend: &B,
    caller: &B::CallerCtx,
    args: Value,
) -> Result<(Value, Vec<Invocation>), JmapError>

Generic */get handler (RFC 8620 §5.1).

Fetches objects by id (or all objects when ids is absent or null) and returns the standard get response shape.