[codex] Cover request id echo and correlation

[pq198363-ops]

Summary

• Adds focused Supertest coverage for X-Request-Id echo, minting, and error-body correlation.
• Covers caller-provided ids, missing ids, overlong ids, supplied-id 404 correlation, and minted-id 404 correlation.
• Keeps production behavior unchanged.

Closes #154

RED / GREEN evidence

• RED coverage gap: after a clean build, node --test dist/request-id-echo.test.js failed because the focused request-id coverage file did not exist.
• GREEN focused: npm run build; $env:NODE_ENV='test'; node --test dist/request-id-echo.test.js passed 5 tests.

Verification

• npx prettier --check src/request-id-echo.test.ts
• npm run lint
• clean dist/, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js (94 tests, 11 suites, 94 pass, 0 fail)
• git diff --check

Note: on Windows PowerShell I used the direct $env:NODE_ENV='test'; node --test ... equivalent after npm run build because the package npm test script uses POSIX-style NODE_ENV=test syntax.