fix: Change set arithmetic for object store sensor

Update the set arithmetic for object store sensors to use the `-` operator to ensure
that only keys that are present in the bucket but not the cursor are retrieved. Also
ensure that the cursor is loaded as Python by deserializing from JSON.

src/ol_orchestrate/sensors/sync_gcs_to_s3.py

@@ -40,7 +40,7 @@ def check_new_gcs_assets_sensor(  # noqa: PLR0913
         for file in storage_client.list_blobs(bucket, prefix=bucket_prefix)
         if object_filter_fn(file.name)
     }
-    new_files: set[str] = bucket_files.difference(set(context.cursor or []))
+    new_files: set[str] = bucket_files - set(json.loads(context.cursor or "[]"))
     if new_files:
         context.update_cursor(json.dumps(list(bucket_files)))
         yield RunRequest(
@@ -75,7 +75,7 @@ def check_new_s3_assets_sensor(  # noqa: PLR0913
         )
         if object_filter_fn(obj["Key"])
     }
-    new_files: set[str] = bucket_files.difference(set(context.cursor or []))
+    new_files: set[str] = bucket_files - set(json.loads(context.cursor or "[]"))
     if new_files:
         context.update_cursor(json.dumps(list(bucket_files)))
         yield RunRequest(