Core API: Newsletter required fields

🔍 Problem Statement

The Core API does not enforce or document required fields when creating or updating a newsletter. Developers encounter validation errors without clear guidance on which fields are mandatory, slowing integration development.

💡 User Story

As a developer, I want newsletter required fields clearly enforced and documented in the Core API, so that I can build integrations correctly without trial-and-error field discovery.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given the newsletter creation/update API endpoint, when a developer omits a required field, then the API returns a clear error specifying which field is missing — and the API documentation lists all required fields explicitly.