feat(fusion_shipping): UPS bill-receiver clarity + controllable commercial invoice

- Relabel UPS 'Bill My Account' -> "Bill recipient's UPS account" with clear
  help (it bills the customer's own UPS account; $0 shipping line; falls back to
  Bill Shipper when the customer has no account on file).
- Improve the customer 'UPS Account Number' field help (stored per-customer,
  auto-recalled at ship time for Bill Receiver).
- Add ups_rest_documentation_type setting (No / UPS commercial invoice) on the
  UPS REST carrier, mirroring FedEx. Default 'invoice' preserves the existing
  auto-generate-on-international behaviour; gate require_invoice on it so it can
  be turned off. Surfaced on the UPS REST config page.

Validated live on entech (UPS production): CA->US shipment generated the label
+ a 60KB commercial invoice PDF (country of origin auto = CA, HS code applied),
then voided. Bill Receiver request confirmed accepted by UPS.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_shipping/__manifest__.py

@@ -1,6 +1,6 @@
 {
     "name": "Fusion Shipping",
-    "version": "19.0.1.5.0",
+    "version": "19.0.1.6.0",
     "category": "Inventory/Delivery",
     "summary": "All-in-one shipping integration — Canada Post, UPS, FedEx, DHL Express. "
     "Live pricing, label generation, shipment tracking, and multi-package support.",