Hotfix for issue exposed in HA 2026.3

• During configuration our protocol version field has been a mix of
  numbers and the string "auto". This used to work in previous HA
  versions, but in 2026.3 they have consolidated some front end
  components and the requirement for dropdowns has become more
  strict. Now we use strings in the config flow, and convert back to
  numbers internally. #4645, #4646 by @kongo09