Function create_bos_session 

pub async fn create_bos_session(
    infra: &InfraContext<'_>,
    token: &str,
    bos_session: BosSession,
) -> Result<BosSession, Error>

Submit a [BosSession] previously built by validate_and_prepare_template_session.

This is a thin wrapper kept so the handler stays a one-liner and the validate / create steps remain separate testable units.