fix type issues

Signed-off-by: varun-edachali-dbx <varun.edachali@databricks.com>

Author: varun-edachali-dbx

src/databricks/sql/backend/sea/result_set.py

@@ -27,6 +27,8 @@
 class SeaResultSet(ResultSet):
     """ResultSet implementation for SEA backend."""
 
+    backend: SeaDatabricksClient
+
     def __init__(
         self,
         connection: Connection,
@@ -83,10 +85,6 @@ def __init__(
             arrow_schema_bytes=execute_response.arrow_schema_bytes,
         )
 
-        assert isinstance(
-            self.backend, SeaDatabricksClient
-        ), "SeaResultSet must be used with SeaDatabricksClient"
-
     def _convert_complex_types_to_string(
         self, rows: "pyarrow.Table"
     ) -> "pyarrow.Table":