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>

packages/core-ui/package.json

@@ -27,9 +27,11 @@
     "@testing-library/jest-dom": "^6.5.0",
     "@testing-library/react": "^16.0.0",
     "@testing-library/user-event": "^14.5.0",
+    "@types/jest-axe": "^3.5.9",
     "@types/react": "^19.0.0",
-    "jsdom": "^25.0.0",
     "@vitest/coverage-v8": "^3.0.0",
+    "jest-axe": "^10.0.0",
+    "jsdom": "^25.0.0",
     "vitest": "^3.0.0"
   }
 }