fix(highlight): refresh highlight attrs before message output #25737

Problem:
When MsgArea highlight is changed, the next message may flash
and disappear because msg_start() renders with stale highlight attributes.

msg_puts_len() uses HL_ATTR(HLF_MSG) to render message text, which
happens before update_screen() calls highlight_changed().
So the message is rendered with outdated attrs.

Solution:
Call highlight_changed() in msg_start().

src/nvim/message.c

@@ -1655,6 +1655,10 @@ void msg_start(void)
     need_fileinfo = false;
   }
 
+  if (need_highlight_changed) {
+    highlight_changed();
+  }
+
   if (need_clr_eos || (p_ch == 0 && redrawing_cmdline)) {
     // Halfway an ":echo" command and getting an (error) message: clear
     // any text from the command.