Create or update a contact

Creates a new contact or updates an existing one (upsert).

Lookup order:

1. If _id is provided, finds by ID.
2. Otherwise finds by first email address.
3. If no match is found, creates a new contact.

Body formats — either native or legacy (capitalized keys) are accepted:

Native: firstname, lastname, email (string or array), phone (array of {number, type}), typeId, contactType, isCompany, title, company, imageUrl, customFields

Legacy: "First Name", "Last Name", Email, "Mobile Phone", "Work Phone", "Home Phone", "Contact Type", Title, Company

Returns the enriched contact with custom fields populated by their type definitions.