Function cmd_config_update 

pub async fn cmd_config_update(
    client: &VtaClient,
    label_prefix: &str,
    vta_name: Option<String>,
    public_url: Option<String>,
) -> Result<(), Box<dyn Error>>

Patch configuration keys.

vta_did is deliberately not a parameter: the VTA’s own identity is set at setup and is immutable at runtime, so there is no flag to attempt it with. A caller that names it anyway (over the wire) is answered with a rejection, which this command prints — the operator learns the rule rather than silently re-pointing the agent’s identity, which is what the pre-canonical surface did.