Use ActivitySerializationContext when describing a Standalone Activity (#1583)

* Use ActivitySerializationContext when describing a Standalone Activity

* remove unnecessary check

Author: VegetarianOrc

temporalio/client/_impl.py

@@ -698,9 +698,14 @@ async def describe_activity(
             long_poll_token=resp.long_poll_token or None,
             namespace=self._client.namespace,
             data_converter=self._client.data_converter.with_context(
-                WorkflowSerializationContext(
+                ActivitySerializationContext(
                     namespace=self._client.namespace,
-                    workflow_id=input.activity_id,  # Using activity_id as workflow_id for activities not started by a workflow
+                    activity_id=resp.info.activity_id,
+                    activity_task_queue=resp.info.task_queue,
+                    activity_type=resp.info.activity_type.name,
+                    workflow_id=None,
+                    workflow_type=None,
+                    is_local=False,
                 )
             ),
         )