[Tooltip]: improve a11y

I can't quite tell which aria role we should use, but it would seem we need something to indicate when it's open, and what it's purpose is. Would it be `aria-haspop="dialog"` and `aria-expanded="true|false"`?

Start by locating the Tooltip component and reviewing its current rendered semantics and interaction behavior. Check the relevant ARIA guidance for tooltip purpose and open-state communication, then verify the result with an accessibility-focused test or browser inspection.

Written by the indexing model from the issue text.