Omnisend plugin. Update API

Omnisend API v3 has been deprecated. V3 remains operational but no longer receives new features or improvements; critical and security issues will still be addressed.

Sunset - 31 December 2027. V3 will be turned off. After this date, any integration calling v3 will stop syncing contacts, products, categories, and events, and carts/orders will no longer be accessible via the API.

We should migrate our integration to the latest API version, v2026-03-15. Here is a step-by-step migration guide covers every change.

Key breaking changes to plan for:

• Base URL: https://api.omnisend.com/v3/ becomes https://api.omnisend.com/api/
• Auth header: X-API-KEY: {key} becomes Authorization: Omnisend-API-Key {key}
• New required header: Omnisend-Version: 2026-03-15 on every request
• Carts and orders are no longer readable or writable as resources - cart and order activity is now sent as events (started checkout, added product to cart, placed order, order fulfilled, order canceled, order refunded)
• Pagination for contacts, campaigns, segments, and automations is now cursor-based
• Error responses now follow RFC 9457

v2026-03-15 is also OpenAPI/Swagger-documented, with full campaign CRUD and batch tag operations added - none of which are coming to v3.