Incorrect handling of `drop` for `RefCell` (borrow flag modified during drop)

[Stevengre]

The current handling of drop appears to be incorrect.
In the case of RefCell, the drop operation modifies the internal borrow variable, which is not the intended behavior.

• See: feat: model interior borrows for refcell #786 , which introduces a mock drop for RefCell. However, the current implementation might be semantically unsound.

We plan to address this by introducing a spl-token specific cheatcode in a separate PR without touching the drop, along with fix: drop to handle drop generally not just for Refcell.