test: cover rate limiter and pause guard

[pq198363-ops]

Summary

• add dedicated protective middleware coverage in src/middleware.test.ts
• cover the 60-request rate-limit boundary and the next-request 429 rate_limited response
• assert Retry-After, message, and requestId on rate-limit failures
• cover pause guard behavior for blocked writes, allowed GET/HEAD/OPTIONS reads, admin status, and unpause recovery

Closes #44

Validation

• npx prettier --check src/middleware.test.ts
• npm run lint
• git diff --check
• npm run build
• $env:NODE_ENV='test'; node --test dist/middleware.test.js (2 tests, 1 suite)
• clean dist, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js (91 tests, 11 suites)

Notes

This is test-only. The rate-limit test temporarily enables runtime rate limiting within the focused test and restores NODE_ENV afterward, so the rest of the suite still uses the normal test environment.