Use ActivitySerializationContext when describing a Standalone Activity

Author: VegetarianOrc

temporalio/client/_impl.py

@@ -693,14 +693,22 @@ async def describe_activity(
             metadata=input.rpc_metadata,
             timeout=input.rpc_timeout,
         )
+        activity_type = (
+            resp.info.activity_type.name if resp.info.HasField("activity_type") else ""
+        )
         return await ActivityExecutionDescription._from_execution_info(
             info=resp.info,
             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=activity_type,
+                    workflow_id=None,
+                    workflow_type=None,
+                    is_local=False,
                 )
             ),
         )