fix static type check

Signed-off-by: Sai Shree Pradhan <saishree.pradhan@databricks.com>

Author: saishreeeee

src/databricks/sql/common/feature_flag.py

@@ -159,6 +159,8 @@ def get_instance(cls, connection: "Connection") -> FeatureFlagsContext:
         """Gets or creates a FeatureFlagsContext for the given connection."""
         with cls._lock:
             cls._initialize()
+            assert cls._executor is not None
+
             # Use the unique session ID as the key
             key = connection.get_session_id_hex()
             if key not in cls._context_map: