user_switch_response.dart was left holding a single 13-line function after
UserSwitchResponse's decorative fields were dropped, so the filename no
longer described its contents and it sat at lib/models/ root while every
other Plex model lives in lib/models/plex/.
Renamed to lib/models/plex/plex_switch_response.dart, with the test moved
alongside the other plex_*_test.dart files. The parser stays public so the
#1488 drift characterization tests keep exercising it directly.