Function ensured_existing_row 

fn ensured_existing_row(
    store: &LogicalStore,
    entity: &str,
    unique_key: &[String],
    values: &BTreeMap<String, WriteValue>,
    steps: &mut Vec<Mutation>,
) -> Result<Option<Map<String, Value>>, FeatureRuntimeError>

Captures an ensured row through the interface and guards its snapshot before writes.