fix(ui): cmdline_show is not fired after clearing the screen (#36451)

Problem:  Resize events during startup may clear an active external
          cmdline, which is then not redrawn.
          UI2 VimResized autocommand does not work.
          UI2 message appearance may be altered by inherited window
          options. The message separator uses the wrong fillchar.

Solution: Unset cmdline_was_last_redrawn when clearing the screen, such
          that cmdline_show is re-emitted.
          Ensure set_pos function is called without arguments.
          Ensure such options are unset. Use 'fillchars'->msgsep.

runtime/lua/vim/_extui.lua

@@ -126,7 +126,9 @@ function M.enable(opts)
 
   api.nvim_create_autocmd({ 'VimResized', 'TabEnter' }, {
     group = ext.augroup,
-    callback = ext.msg.set_pos,
+    callback = function()
+      ext.msg.set_pos()
+    end,
     desc = 'Set cmdline and message window dimensions after shell resize or tabpage change.',
   })