fix: eBay 25101 Invalid <ShippingPackage> — never send package types eBay US rejects

Regression from the audit overhaul (#32): the per-item-class package
profiles set packageType MAILING_BOX for boxed classes (shoes, coats,
electronics…). That value exists in the Inventory API schema but eBay US
rejects it at publish with 25101 "Invalid <ShippingPackage>". Only weight
and dimensions drive calculated-shipping cost anyway.

- Every profile now ships PACKAGE_THICK_ENVELOPE (eBay US's generic
  'Package (or thick envelope)' — the value the app always used before);
  profiles differ only by weight/dimensions
- New recovery at both inventory-create and publish stages: on 25101,
  snap the type to the safe value and retry; if it was already safe,
  drop packageWeightAndSize entirely (flat-rate publishes fine without
  it; calculated policies then surface eBay's clearer 25020)
- 18 new regression tests (110 total), incl. one asserting no category
  profile can ever emit a different package type