Fix hide_code not taking effect on kernel-created cells (#8926)

Cells created via code_mode with `hide_code=True` and non-empty code
showed their editor anyway because they were marked "untouched," which
overrides `hide_code` in the render logic. That override only makes
sense for empty cells the user just created, so gate it on `!code` as
well.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: manzt

frontend/src/core/cells/cells.ts

@@ -247,9 +247,10 @@ const {
         [newCellId]: createRef(),
       },
       scrollKey: autoFocus ? newCellId : null,
-      untouchedNewCells: hideCode
-        ? new Set([...state.untouchedNewCells, newCellId])
-        : state.untouchedNewCells,
+      untouchedNewCells:
+        hideCode && !code
+          ? new Set([...state.untouchedNewCells, newCellId])
+          : state.untouchedNewCells,
     };
   },
   moveCell: (