fix(a11y): name NotificationPanel drawer and announce new items

[ajirthan]

Summary

• Name the NotificationPanel drawer with default aria-label="Notifications" (overridable; empty/whitespace falls back)
• Announce list growth while open via a polite live region; support consumer liveAnnouncement
• Add regression tests, LiveAnnouncements Storybook story, and accessibility docs updates

Addresses wso2/oxygen-ui#560 (parent epic #557).

Commits

1. Drawer aria-label + live-region infrastructure
2. List-growth announcements while open
3. Accessibility regression tests
4. Docs + LiveAnnouncements story

Test plan

• pnpm --filter @wso2/oxygen-ui exec vitest run src/components/accessibility.test.tsx
• Storybook → App Elements / Notification Panel → LiveAnnouncements (add item + sync status)
• Confirm Storybook a11y panel stays green aside from existing color-contrast (A11y: Brand primary #FF7300 fails WCAG AA 4.5:1 text contrast (Classic/WSO2 themes) wso2/oxygen-ui#558) exception

---

Note

Medium Risk
New public props and context API require consumers to call setLiveAnnouncement for new items; behavior is well-tested but changes how notification UX should be wired for screen readers.

Overview
NotificationPanel now gives the drawer a default accessible name (aria-label="Notifications", overridable; empty/whitespace keeps the default) via drawer slotProps.paper, and adds a visually hidden polite role="status" live region for status updates.

New arrivals are not inferred from list child count (tabs/filters would false-positive). Apps publish explicitly with the liveAnnouncement prop or useNotificationPanel().setLiveAnnouncement, only while the panel is open; repeated identical text remounts the live region so assistive tech can hear it again. NotificationPanelContext also exposes open for compound children.

Docs and Storybook are updated (including a LiveAnnouncements story); wso2#560 is resolved and removed from follow-ups. accessibility.test.tsx adds regression coverage for naming, live-region behavior, and edge cases (close, reopen, persistent closed).

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[ajirthan]

@BugBot review

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Cursor Bugbot has reviewed your changes using high effort and found 3 potential issues.

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[ajirthan]

Superseded by upstream PR: wso2#568