add query system test

Author: Linchin

tests/system/test_client.py

@@ -1945,6 +1945,16 @@ def test_dml_statistics(self):
         assert query_job.dml_stats.updated_row_count == 0
         assert query_job.dml_stats.deleted_row_count == 3
 
+    def test_query_w_picosecond_timestamp(self):
+        query_job = Config.CLIENT.query(
+            "SELECT CAST(\"2025-10-20\" AS TIMESTAMP(12)) as time_pico;",
+        )
+        expected_schema = [
+            bigquery.SchemaField("time_pico", "TIMESTAMP", mode="NULLABLE", timestamp_precision=12),
+        ]
+        iterator = query_job.result()
+        self.assertEqual(iterator.schema, expected_schema)
+
     def test_transaction_info(self):
         table_schema = (
             bigquery.SchemaField("foo", "STRING"),