test: add Wix product option fixture

[RujulaSharma]

Summary

Adds a synthetic Wix Stores Catalog V3 product option fixture for issue #423.

Included

• Synthetic Size and Color product options
• Four synthetic variant combinations
• Expected Wix -> Posnic variant mapping
• SKU -> itemid mapping
• Barcode -> barcode_id mapping
• Missing SKU and barcode behavior
• Official Wix Stores API documentation references
• Tests covering the fixture and integration boundary

Scope

This is limited to fixture and documentation work.

• No live Wix API calls
• No Wix authentication or installation flow
• No production connector implementation
• No real merchant or customer data

Validation

• node --test tests/wix-product-option-fixture.test.js
• 7 tests passed
• git diff --cached --check passed

Closes #423

[sridharkalaibala]

The Wix product-option fixture is synthetic, clearly distinguishes expected future mapping from a live connector, and covers absent identifiers safely. Verified git diff --check; all published checks are green.

[github-actions]

Merged to develop. Anyone can test this - you do not need write access.

Try it at https://develop.posnic.io, or run it yourself:

git fetch origin develop && git checkout develop
npm install && npm --prefix api install
npm run dev   # then http://localhost:3000

When you have tested it, say what you did and what happened, and set
QA passed or QA failed. If you cannot set labels, just comment -
a maintainer will.

Reporting that something is broken is as useful as fixing it. It is
better found here than by a shopkeeper.