Function build_execution_bundle 

pub fn build_execution_bundle(
    bundle_id: BundleId,
    plan: &ExecutionPlan,
    selected_variant_id: Option<VariantId>,
    selections: BTreeMap<String, SelectionDecision>,
    refit_artifacts: Vec<RefitArtifactRecord>,
) -> Result<ExecutionBundle, DagMlError>