Add notifications behaviour unit tests

[ajirthan]

Part of #598

Parent

#598 — first-party Oxygen UI unit tests (epic)

What to build

Colocated behaviour tests for notifications: NotificationPanel slots, useNotifications, and NotificationBanner — list, empty, dismiss, and mark-read.

Acceptance criteria

• NotificationPanel slots that change structure or behaviour are locked (including empty)
• useNotifications locks add, dismiss, and mark-read at the public hook API
• NotificationBanner locks show / dismiss / severity at the public API (not paint)
• Tests go through the shared render-with-theme helper from Add test contract and render-with-theme helper #601
• The existing accessibility file is left in place
• The library Vitest suite passes

Blocked by

• Add test contract and render-with-theme helper #601