Function create_test_query_sql 

fn create_test_query_sql(
    flattened_target_name: &str,
    at_time: Option<&str>,
) -> String

Create the test assertion query that returns failures.

Returns rows with a ‘status’ column indicating the failure mode:

• ‘MISSING’: Expected rows not found in actual results
• ‘UNEXPECTED’: Actual rows not found in expected results

Empty result means the test passed.

If at_time is provided, the query includes an AS OF clause to set the value of mz_now() during test execution.