curl -X PUT "https://app.whattalk.ai/api/user/leads/123" \
  -H "Authorization: Bearer YOUR_API_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "campaign_id": 456,
    "phone_number": "+1234567890",
    "variables": {
      "name": "John Doe",
      "company": "Acme Corp"
    }
  }'

{
  "message": "Lead updated successfully"
}

{
  "message": "Lead not found"
}

{
  "message": "Campaign not found"
}

{
  "message": "Invalid phone number"
}

{
  "message": "Validation failed",
  "errors": {
    "campaign_id": ["The selected campaign does not exist"],
    "status": ["This status is assigned automatically and cannot be set manually"],
    "variables": ["Variables must be an array"]
  }
}

{
  "message": "Invalid fields: invalid_field, another_invalid_field"
}