/** Case 2: Key conflict (without .overwrite) → Overwrite with value from responseDataMap */
    public void test_mergeResponseData_keyConflict() {
        Map<String, Object> dataMap = new HashMap<>();
        dataMap.put("x", "X0");
        Map<String, Object> responseDataMap = new HashMap<>();
        responseDataMap.put("x", "X1");

        indexUpdateCallback.mergeResponseData(dataMap, responseDataMap);

        assertEquals(1, dataMap.size());