Function resolve_target_nodes 

pub async fn resolve_target_nodes(
    backend: &StaticBackendDispatcher,
    shasta_token: &str,
    hosts_expression: Option<&str>,
    hsm_group_name_arg_opt: Option<&str>,
    settings_hsm_group_name_opt: Option<&str>,
) -> Result<Vec<String>, Error>

Resolve target nodes from either a hosts expression, an explicit HSM group name, or the settings-level HSM group.

Priority order:

1. hosts_expression — parsed and validated via resolve_hosts_expression.
2. hsm_group_name_arg_opt — the CLI --hsm-group argument; validated for access via crate::server::common::authorization::get_groups_names_available, then expanded to member xnames.
3. settings_hsm_group_name_opt — the group configured in the environment or config file; same treatment as (2).

Returns a sorted, deduplicated Vec<String> of xnames.