Use ActiveCampaign's Contacts API to sync a contact list with tags and custom field values, implementing upsert logic using the sync endpoint and handling field type validation

Steps

1. POST /api/3/contact/sync with a JSON body containing 'contact' object (email, firstName, lastName, phone) and 'fieldValues' array of objects with 'field' (field ID as string) and 'value' to upsert the contact
2. Retrieve field IDs by GET /api/3/fields to build a mapping from field names to IDs before syncing
3. Apply tags by POST /api/3/contactTags with 'contactTag' object containing 'contact' (contact ID from sync response) and 'tag' (tag ID); retrieve tag IDs via GET /api/3/tags?search={tagName}
4. Handle the 'fieldValues' validation: date fields require YYYY-MM-DD format, checkbox fields require '1'/'0' strings, and multi-select fields require pipe-delimited values (e.g. 'optionA||optionB')
5. Verify the upserted contact by GET /api/3/contacts/{id}?include=fieldValues,contactTags to confirm all fields and tags were applied