feat(mat-expansion-panel): Add a design token for adding a border

### Feature Description

Please consider adding a design token for a border.

### Use Case

_No response_

Start by locating the mat-expansion-panel implementation and its existing design-token definitions. Check how related Material components expose border tokens and what tests cover token customization. Done means agreeing on the token's name, supported value, generated styling, and test coverage; the issue provides no specific files or use case.

Written by the indexing model from the issue text.