Function backfill_target_runtime 

pub fn backfill_target_runtime(
    session_id: &str,
    target_template_id: &str,
    runtime: &TargetRuntimeSettings,
) -> Result<Option<TargetRuntimeSettings>>

Fill a missing snapshot without overwriting a concurrent writer. A deleted or retargeted session returns None; its stale controller copy can be dropped by reload instead of preventing every subsequent reload.