chore: use get_handle() instead of private session attribute in client

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

Author: varun-edachali-dbx

src/databricks/sql/session.py

@@ -134,7 +134,7 @@ def get_id_hex(self) -> str:
     def close(self) -> None:
         """Close the underlying session."""
         logger.info(f"Closing session {self.get_id_hex()}")
-        if not self.open:
+        if not self.is_open:
             logger.debug("Session appears to have been closed already")
             return