test: cover error handler responses

[pq198363-ops]

Summary

• add dedicated src/errors.test.ts coverage for oversized JSON bodies, malformed JSON bodies, and unknown route fallbacks
• assert every covered error response includes error, message, and requestId, without stack/internal path leakage
• move request id attachment before express.json() so parser-generated 413 and malformed JSON errors can include the same request correlation field

RED evidence

Before the middleware-order fix, the focused suite failed because both parser-generated errors omitted requestId:

• 413 payload_too_large for oversized JSON bodies
• 500 internal_error for malformed JSON bodies

Verification

• npx prettier --check src/errors.test.ts src/middleware/index.ts
• npm run lint
• git diff --check
• $env:NODE_ENV='test'; node --test dist/errors.test.js
• $env:NODE_ENV='test'; node --test dist/health.test.js dist/routes/operational.test.js
• clean dist/, npm run build, $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js

Full clean test output summary:

• tests: 93
• suites: 11
• pass: 93
• fail: 0

Closes #47