fixes DEVEX-375

Replaces getByRole with async findByRole, which returns a promise / rejects if no matching element is found after a default timeout of 1000ms

Co-written with Coder Agents. Relevant chat responses:

[I couldn't repro locally, so I inquired if there was a commit/PR that fixed the flake within the past 3 weeks]

No, this flake has not been fixed. There have been zero commits to CreateTokenPage, CopyButton, CodeExample, or useClipboard.ts since the failing CI run (13bf0e1, May 20).

getByRole is synchronous, so it doesn't wait for the success modal (containing the "Copy code" button) to render after the createToken() mutation resolves. When the mutation is slow, the DOM still shows the form (Cancel / Create token), and the query fails.