Function exec 

pub async fn exec(
    infra: &InfraContext<'_>,
    token: &str,
    image_id: &str,
) -> Result<String, Error>

Launch an ephemeral IMS customize container against image_id and return its SSH hostname.

The caller’s preferred username is read from the JWT and used to look up their registered SSH public key in IMS. If no key is registered, returns NotFound with a message pointing the user at platform admins. The hostname is plucked from the IMS response at /ssh_containers/0/connection_info/customer_access/host; a missing field is reported as MissingField rather than a generic error so operators can tell schema drift from real failures.

Errors

• [Error::JwtMalformed] (via wire_conv::to_backend) when the JWT carries no preferred_username.
• [Error::BadRequest] if the Shasta HTTP client cannot be built or the IMS customize job submission fails.
• [Error::NotFound] when the caller has no SSH public key registered in IMS.
• [Error::MissingField] when the IMS response is missing the server-generated key id or the SSH container host field.