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, validate target group exists, and add nodes to the HSM group.

Returns (xnames_resolved, updated_member_list).