Function add_nodes_to_group 

pub async fn add_nodes_to_group(
    infra: &InfraContext<'_>,
    token: &str,
    target_hsm_name: &str,
    hosts_expression: &str,
) -> Result<(Vec<String>, Vec<String>), Error>

Resolve hosts_expression and add the resulting nodes to the existing HSM group target_hsm_name.

The target group must already exist (an explicit NotFound is returned rather than the backend’s opaque error). An empty resolution is rejected with BadRequest. Returns the resolved xnames alongside the group’s sorted, post-update membership.