Support multi-zone service rate rules

[roncodes]

Summary

Adds FleetOps Data model/serializer support for generic multi_zone_distance service-rate rules.

Changes

• Adds service-rate-fee fields for zone/service-area rule references, labels, priority, and fallback behavior.
• Adds isMultiZoneDistance and rule helpers on the service-rate model.
• Preserves embedded zone/service-area data on fee serialization.
• Adds unit coverage for multi-zone rule sorting and default rule creation.

Validation

• npm run lint:js -- addon/models/service-rate.js addon/models/service-rate-fee.js addon/serializers/service-rate.js addon/serializers/service-rate-fee.js tests/unit/models/service-rate-test.js
• git diff --check

Note: targeted Ember test build succeeded, but Testem execution is currently blocked by the local CommonJS/ESM execa incompatibility.