fix(coderd/x/chatd): archive chat hard-interrupts active stream (#23758)

Archiving a chat now transitions pending or running chats to waiting
before setting the archived flag. This publishes a status notification
on `ChatStreamNotifyChannel` so `subscribeChatControl` cancels the
active `processChat` context via `ErrInterrupted` — the same codepath
used by the stop button.

The `processChat` cleanup also skips queued-message auto-promotion when
the chat is archived, so archiving behaves like a hard stop rather than
interrupt-and-continue.

Relates to https://github.com/coder/coder/issues/23666

coderd/exp_chats.go

@@ -1641,9 +1641,9 @@ func (api *API) patchChat(rw http.ResponseWriter, r *http.Request) {
 		}
 
 		var err error
-		// Use chatDaemon when available so it can notify active
-		// subscribers. Fall back to direct DB for the simple
-		// archive flag — no streaming state is involved.
+		// Use chatDaemon when available so it can interrupt active
+		// processing before broadcasting archive state. Fall back to
+		// direct DB when no daemon is running.
 		if archived {
 			if api.chatDaemon != nil {
 				err = api.chatDaemon.ArchiveChat(ctx, chat)