test: cover usage recording paths

[pq198363-ops]

Summary

• add dedicated usage recording coverage in src/usage.test.ts
• cover first-write, accumulation, read-back, and unseen-pair zero behavior
• cover deterministic saturation at Number.MAX_SAFE_INTEGER
• cover disabled-service 409 service_disabled without increasing the usage total
• assert the standard 400 invalid_request shape with message and requestId across validation failures

Closes #98

Validation

• npx prettier --check src/usage.test.ts
• npm run lint
• git diff --check
• npm run build
• $env:NODE_ENV='test'; node --test dist/usage.test.js (16 tests, 1 suite)
• clean dist, then npm run build
• $env:NODE_ENV='test'; node --test dist/*.test.js dist/**/*.test.js (105 tests, 11 suites)

Notes

This is intentionally test-only. The focused suite did not expose a production behavior bug.