cover_json_() did not serialize the assumed_state flag in the DETAIL_ALL
JSON, unlike switch_json_() which already emits it. Consumers of the web
server JSON/SSE API could not tell whether a cover's state is assumed,
causing a web vs native-API inconsistency (the native API exposes the
flag, so Home Assistant behaves correctly, but the web UI does not).
Emit assumed_state from the cover's traits, matching the switch.