refactor: move the Plex switch-token parser in with the Plex models

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.

lib/services/plex_auth_service.dart

@@ -8,7 +8,7 @@ import '../exceptions/media_server_exceptions.dart';
 import '../models/plex/plex_user_profile.dart';
 import '../models/plex/plex_home.dart';
 import '../models/plex/plex_home_user.dart';
-import '../models/user_switch_response.dart';
+import '../models/plex/plex_switch_response.dart';
 import '../utils/app_logger.dart';
 import '../utils/device_identity.dart';
 import '../utils/endpoint_race.dart';

test/models/plex_switch_response_test.dart

@@ -1,5 +1,5 @@
 import 'package:flutter_test/flutter_test.dart';
-import 'package:plezy/models/user_switch_response.dart';
+import 'package:plezy/models/plex/plex_switch_response.dart';
 
 /// A realistic `/api/v2/home/users/{uuid}/switch` 201 body using the
 /// July 2026 wire shape where profile language lists are CSV strings (#1488).