test: update tests for client credentials auth

- Add auth.test.ts: request/parse token, cache hit, skew-window
  refresh, invalidate, single-flight concurrency
- graphql.test.ts: switch to tokenProvider; add 401 → invalidate →
  retry case and max-retries-exhausted case
- config.test.ts: update to SHOPIFY_API_KEY / SHOPIFY_API_SECRET
- tools.test.ts: use staticTokenProvider helper