If startSessionStream(sid) runs while the tab is ALREADY hidden (session loaded/
restored in a background tab), it set _sessionStreamSessionId then returned at the
hidden-skip WITHOUT recording _sessionStreamHiddenSid — so the visibility handler
(which reopens only from the holder) never reattached, silently dropping
bg_task_complete/server_turn_started until the next explicit startSessionStream.
Set _sessionStreamHiddenSid = sid on the hidden-skip return. Adds a regression test.
