fix(ui): only internal messages are unsafe #37462

Problem:  Fast context for msg_show event inhibits vim.ui_attach from
          displaying a stream of messages from a single command.

Solution: Remove fast context from msg_show events emitted as a result
          of explicit API/command calls. The fast context was originally
          introduced to prevent issues with internal messages.

runtime/lua/vim/_extui/shared.lua

@@ -5,6 +5,7 @@ local M = {
   ns = api.nvim_create_namespace('nvim._ext_ui'),
   augroup = api.nvim_create_augroup('nvim._ext_ui', {}),
   cmdheight = vim.o.cmdheight, -- 'cmdheight' option value set by user.
+  redrawing = false, -- True when redrawing to display UI event.
   wins = { cmd = -1, dialog = -1, msg = -1, pager = -1 },
   bufs = { cmd = -1, dialog = -1, msg = -1, pager = -1 },
   cfg = {