[Bug]: dissmissable attribute on dialog hides slot=button

### Code of conduct

- [X] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-dialog

### Expected behavior

`slot=button` content to be visible when `dissmissable` attribute is set on `sp-dialog`

### Actual behavior

`slot=button` content is hidden with `display: none;`

### Screenshots

_No response_

### What browsers are you seeing the problem in?

_No response_

### How can we reproduce this issue?

1. Go to '...'
2. Click on '....'
3. Scroll to '....'
4. Check console
5. See error

### Sample code or abstract reproduction which illustrates the problem

_No response_

### Severity

None

### Logs taken while reproducing problem

_No response_

Start by inspecting the sp-dialog implementation and the styling or rendering path for dismissable dialogs and slot=button content. Reproduce the issue with dismissable set, then verify that the slotted button remains visible while the dialog's dismiss control still works.

Written by the indexing model from the issue text.