test(core-ui): add axe-core a11y assertions to CookieConsentBanner

Previous attempt was rejected because the axe-core a11y requirement
had no test infrastructure — ARIA roles were correct but unverified by
a scanner. This adds jest-axe (approved via library-decision trace) and
asserts toHaveNoViolations() for both modal and banner variants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

coverage/summary.json

@@ -1,6 +1,6 @@
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