test: cover usage rollups and exports

[pq198363-ops]

Summary

• adds dedicated src/aggregations.test.ts coverage for usage rollups and export endpoints
• seeds multiple agents and services, then verifies agent totals, per-agent usage, service totals, service consumers, and top-agent ordering
• covers top-agent limit clamping, empty agent/service rollups, CSV comma/quote/newline escaping, the CSV header row, and JSON exportedAt plus all (agent, serviceId, total) tuples

Verification

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

Full test output summary:

• tests: 92
• suites: 11
• pass: 92
• fail: 0

Closes #46