Extract device attribute addition into helper function to reduce
cyclomatic complexity from 10 to 9:
- callsign_json.ex: Extract add_device_attributes/2 helper
Moves device.model, device.vendor, device.contact, and device.class
attribute additions into separate function
This reduces complexity by isolating the && operators in a dedicated
helper function.
Fixes 1 Credo cyclomatic complexity issue (10 remaining).
