Function flatten_usage_columns

crates/ccql/src/sql/composite_storage.rs:359–365

Inject flattened `model` / `usage_*` columns for assistant rows. Values come from `message.model` and `message.usage.*`. A column is only added when the source value exists and the key is not already present at the top level of the record (existing JSON keys always win).

(json: &JsonValue, map: &mut BTreeMap<String, Value>)

/// added when the source value exists and the key is not already present at
/// the top level of the record (existing JSON keys always win).
fn flatten_usage_columns(json: &JsonValue, map: &mut BTreeMap<String, Value>) {
    for (key, value) in flattened_usage_fields(json) {
        if !map.contains_key(key) {
            map.insert(key.to_string(), json_value_to_glue_value(value));
        }
    }
}

/// Convert a todo JSON object to a DataRow
fn todo_json_to_data_row(