Update custom fields for a contact using any of the following identifiers:

• contactId: Direct contact ID (takes highest priority)
• phone: Phone number with country code (e.g., "919876543210")
• email: Email address (case-insensitive lookup)

At least one identifier must be provided. If multiple are provided, the lookup priority is: contactId > phone > email.

This endpoint is useful when you don't have the contact ID readily available but have the phone number or email from your system.

{
  "contactId": 12345,
  "phone": "919876543210",
  "email": "user@example.com"
}