- Fix production bug in send_whatsapp_message: access 'instance_url' instead
of 'server_url' to match SendMessageScheme TypedDict definition
- Update webhook test to use test_app with in-memory dependencies
- Mock send_whatsapp_message to avoid real HTTP calls in tests
- Add assertions to verify correct parameters passed to send_whatsapp_message
Bug: send_whatsapp_message accessed request_data['server_url'] but
SendMessageScheme defines field as 'instance_url', causing KeyError when
webhook tries to send WhatsApp messages.