fix(ui): avoid recursiveness and invalid memory access #28578

Problem:  Calling :redraw from vim.ui_attach() callback results in
          recursive cmdline/message events.
Solution: Avoid recursiveness where possible and replace global "call_buf"
          with separate, temporary buffers for each event so that when a Lua
          callback for one event fires another event, that does not result
          in invalid memory access.

src/nvim/ex_getln.c

@@ -3449,9 +3449,11 @@ void cmdline_screen_cleared(void)
 /// called by ui_flush, do what redraws necessary to keep cmdline updated.
 void cmdline_ui_flush(void)
 {
-  if (!ui_has(kUICmdline)) {
+  static bool flushing = false;
+  if (!ui_has(kUICmdline) || flushing) {
     return;
   }
+  flushing = true;
   int level = ccline.level;
   CmdlineInfo *line = &ccline;
   while (level > 0 && line) {
@@ -3466,6 +3468,7 @@ void cmdline_ui_flush(void)
     }
     line = line->prev_ccline;
   }
+  flushing = false;
 }
 
 // Put a character on the command line.  Shifts the following text to the