[MM-61516] Fix errcheck issues in server/channels/app/plugin_api_test.go (#29220)

server/.golangci.yml

@@ -124,7 +124,6 @@ issues:
         channels/app/platform/web_conn.go|\
         channels/app/platform/web_hub.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/plugin_api_test.go|\
         channels/app/plugin_api_tests/manual.test_http_hijack_plugin/main.go|\
         channels/app/plugin_api_tests/manual.test_serve_metrics_plugin/main.go|\
         channels/app/plugin_api_tests/test_update_user_auth_plugin/main.go|\