antithesis: Fix transaction management

Commit 5216e67d ("bindings/python: Start transaction implicitly in
execute()") fixed transaction management in Python bindings, which means
we now need to execute explicit commit().

antithesis-tests/stress-composer/parallel_driver_insert.py

@@ -44,5 +44,8 @@ for i in range(insertions):
             # Ignore UNIQUE constraint violations
             pass
         else:
+            con.rollback()
             # Re-raise other operational errors
             raise
+
+con.commit()