Fix polymorphic customer type serialization

[roncodes]

Summary

Fixes order and waypoint polymorphic customer serialization so selected customer records serialize with their concrete subtype, contact or vendor, instead of the abstract Ember relationship model, customer.

A recent serializer change removed the fallback to the related record's own polymorphic type attribute. When an order was created with a selected customer model, the order serializer fell back to belongsTo.modelName, producing customer_type fleet-ops:customer. FleetOps does not have a Fleetbase FleetOps Models Customer model, so Laravel later failed resolving the order customer morph relation while building the created webhook payload.

Changes

• Restores the related-record subtype fallback in addon/serializers/order.js.
• Restores the same fallback in addon/serializers/waypoint.js.
• Keeps the existing abstract subtype stripping behavior for values such as customer-contact, customer-vendor, and facilitator-vendor.
• Moves waypoint type resolution behind the null relationship check so null customer relationships still serialize as customer_type null without dereferencing belongsTo.modelName too early.

Why

For customer selector results, the Ember model can be the abstract customer model while the payload includes the concrete subtype in customer_type, for example contact or vendor. The serializer needs to prefer that concrete subtype when present. Otherwise the backend receives fleet-ops:customer, converts it to Fleetbase FleetOps Models Customer, and crashes because the concrete FleetOps model is actually Contact or Vendor.

Verification

• ./node_modules/.bin/prettier --check addon/serializers/order.js addon/serializers/waypoint.js
• ./node_modules/.bin/eslint --no-cache addon/serializers/order.js addon/serializers/waypoint.js
• git diff --check -- addon/serializers/order.js addon/serializers/waypoint.js