Add delivery_bill_id to shipment fulfillment payload

[Marvin-Magmodules]

Channable requires a delivery_bill_id on every shipment update: the number of the delivery note included in the parcel. Marketplaces such as Conrad use it to match their invoice to the physical delivery the customer received.

What it does

Default — the Magento shipment increment ID. Works out of the box for every merchant, no configuration or manual input. It is already printed on Magento's standard packing slip, so the number in the parcel matches the number Channable receives.

ERP override. Merchants who print delivery notes from an ERP (DSN-123) supply their own number through the channable_delivery_bill_id extension attribute on the shipment, either on creation:

POST /rest/V1/order/{orderId}/ship
{
  "tracks": [{"carrier_code": "dhl", "title": "DHL", "track_number": "3S123456789"}],
  "arguments": {"extension_attributes": {"channable_delivery_bill_id": "DSN-123"}}
}

or afterwards, when a shipping extension created the shipment:

POST /rest/V1/shipment
{"entity": {"entity_id": 12, "order_id": 34, "extension_attributes": {"channable_delivery_bill_id": "DSN-123"}}}

Empty falls back to the increment ID, so shipping extensions that know nothing about the field keep working unchanged.

Multi-shipment fixes

• Shipment entries now carry shipment_id. Previously id held the order increment ID, so two shipments of the same order were indistinguishable.
• The order status endpoint merged all shipments into one fulfillment object with +=, silently dropping the tracking data of the second and later shipments. It now returns a fulfillments array with one entry per shipment. The existing fulfillment key is kept and holds the most recent shipment, so the Channable side does not break.

Changes

• Service/Order/Shipping/Fulfillment.php — delivery_bill_id with ERP override and fallback
• Service/Webhook/Shipments.php — shipment_id per entry
• Service/Webhook/OrderStatus.php — one fulfillment entry per shipment
• Plugin/Shipment/DeliveryBillId.php — load and persist the extension attribute
• Plugin/Shipment/DeliveryBillIdArgument.php — accept the value as shipment creation argument
• etc/db_schema.xml — channable_delivery_bill_id on sales_shipment (nullable varchar, instant DDL)
• etc/module.xml — completed the module sequence

Testing

12 new E2E tests in Test/End-2-end/tests/order/delivery-bill-id.spec.ts covering the default, both ERP paths, the fallback and multi-shipment orders. Full suite green on the VPS: 62 passed.

Merchant documentation added in docs/SHIPMENTS.md + docs/SHIPMENTS_NL.md.