Add PredefinedContentHelper for Cloud device API (tapi)

Mirrors the existing terminal/PredefinedContentHelper for the tapi package,
providing feature parity with the Java library for Cloud device API integrations.

- Add src/typings/tapi/predefinedContentHelper.ts with PredefinedContentHelper
  class and DisplayNotificationEvent enum
- Export helper from src/typings/tapi/models.ts
- Export tapi namespace from src/typings/index.ts (Types.tapi.*)
- Add tests in src/__tests__/tapiPredefinedContentHelper.spec.ts
- Document usage in doc/CloudDeviceApi.md under 'Helper classes'

Closes #1720