[collapsible] Exit animation doesn't run when using motion and Tailwind v4

https://codesandbox.io/p/devbox/suspicious-kate-j22y8v

Tailwind v4's reset sets `display: none !important` on the `[hidden]` selector, and the way motion sets styles cannot override `!important`

The workaround is to set `hidden={undefined}` on the panel, it will be unmounted as soon as hidden becomes true anyway:

```jsx

{open && (

}
>
{/* panel content */}

)}

```

## Base UI version

1.0.0-alpha.7