yang: smart_switch dpu_id is defined as string

Passing an integer value in when the yang model is defined as a
string is invalid.  However, its possible the yang model itself
is not proper as it seems to limit the value to 1-7.  However,
such a change would be out of scope for this patch series as
we are just porting to libyang3.

Author: bradh352

src/sonic-yang-models/tests/yang_model_pytests/test_smart_switch.py

@@ -381,9 +381,9 @@ def test_dpu_orchagent_zmq_port(self, yang_model, orchagent_zmq_port, error_mess
 
     @pytest.mark.parametrize(
         "dpu_id, swbus_port, error_message", [
-            (0, 23606, None),
-            (1, 23607, None),
-            (7, 23613, None)]
+            ("0", 23606, None),
+            ("1", 23607, None),
+            ("7", 23613, None)]
         )
     def test_remote_dpu_swbus_port(self, yang_model, dpu_id, swbus_port, error_message):
         data = {