chore: renamed implementations to match new interfaces

feat: updated spec to 2026-01-23

feat: moved processing logic

feat: discount capability

fix: conformance fixes

fix: shipping assignments

feat: place order endpoint

fix: added missing models

feat: fulfillment updates

feat: fulfillment options

feat: implemented fulfillment interfaces

feat: quote validation

feat: update requests

feat: cancel checkout

feat: get checkout endpoint