POS cart: add optional options arg to addLineItem

[vctrchu]

What

Add an optional third argument to the POS cart.addLineItem method so a single call can create a line item and decorate it with line-item properties and cart-level properties:

cart.addLineItem(variantId, quantity, options?: {
  properties?: Record<string, string>;     // line-item properties
  cartProperties?: Record<string, string>; // cart-level properties
});

Introduces the AddLineItemOptions type. Backwards compatible — the third argument is optional, so existing two-argument calls are unchanged.

Why

Cafe-style POS extensions adding a "modified item" (e.g. coffee + modifiers + order metadata) currently chain three extension API calls:

const uuid = await cart.addLineItem(variantId, 1);
await cart.addLineItemProperties(uuid, {modifiers: 'oat milk,extra shot'});
await cart.addCartProperties({orderSource: 'cafe_app'});

Each call awaits its own native SDK call and syncDraftCheckout round-trip — measured at ~3s p50 for the sequential pattern.

[vctrchu]

/snapit

[shopify-github-actions-access]

🫰✨ Thanks @vctrchu! Your snapshots have been published to npm.

Test the snapshots by updating your package.json with the newly published versions:

"@shopify/ui-extensions": "0.0.0-snapshot-20260626012557",
"@shopify/ui-extensions-tester": "0.0.0-snapshot-20260626012557"