A network automation engineer is writing an Ansible playbook to configure a fleet of Nexus 9000 switches. The team wants the playbook to interact with the switches over an HTTPS-based programmatic interface rather than opening interactive SSH CLI sessions, and they have already enabled the 'nxapi' feature on all devices. In the playbook's connection settings, which value must be assigned to the 'ansible_connection' variable so tasks are delivered through the NX-API back end?