test: cover error handler limits

[xx7412421-cloud]

Closes #17

Summary

• add dedicated error handler coverage for JSON bodies over the 100 KiB limit
• assert an exact-limit request reaches route validation instead of the parser limit branch
• assert generic parser failures return the canonical 500 shape with method, path, and requestId
• let the existing rate-limit tests explicitly opt into the limiter so the full suite can validate cleanly under Jest

Validation

• npm test -- errorHandler.test.ts --runInBand
• npm test -- rateLimit.test.ts --runInBand
• npm test -- --runInBand
• npm run lint
• npm run build