fix(api): update topline when flushing with nvim__redraw() (#34346)

Problem:  nvim__redraw may update the screen with an invalid topline.
Solution: Update the topline before calling `update_screen()` (as
          :redraw does).

src/nvim/api/vim.c

@@ -2376,6 +2376,8 @@ void nvim__redraw(Dict(redraw) *opts, Error *err)
   // Redraw pending screen updates when explicitly requested or when determined
   // that it is necessary to properly draw other requested components.
   if (opts->flush && !cmdpreview) {
+    validate_cursor(curwin);
+    update_topline(curwin);
     update_screen();
   }