_SessionBuffer.append_system_message(user_id, session_id, text) injects a
synthetic SystemMessage into the named session slot (creating it if absent).
ContextualBufferProxy closes over user_id + session_id so call sites need
only call proxy.append_system_message(text).
get_session_buffer(user_id, session_id, channel) in device_ws returns a
ContextualBufferProxy, keeping the test-patchable function signature intact.
