Fix transparent websocket recovery for terminals

Close retired websocket connections in stopWSClient so clients reconnect promptly instead of remaining in a stdin-only state with no returning output. Add regression coverage to verify stopWSClient actively disconnects the websocket.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

webterm/server.go

@@ -343,6 +343,9 @@ func (s *LocalServer) stopWSClient(routeKey string, expected *wsClient) {
 		return
 	}
 	close(client.send)
+	if client.conn != nil {
+		_ = client.conn.Close()
+	}
 	<-client.done
 }