Follow-up to the salvaged PR #39439:
- Replace the hardcoded English gateway-auth summary with a
notifications.errors.gatewayAuthFailed i18n key (en/ja/zh/zh-hant + types)
- Fix the malformed ['"'] regex character classes (duplicate quote) in
notifications.ts error matchers
- Add regression test: gateway_auth_failed maps to the gateway auth
summary, provider invalid_api_key still maps to the OpenAI summary (#39365)