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So the message is rendered with outdated attrs.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().

test/functional/ui/embed_spec.lua

@@ -40,7 +40,7 @@ local function test_embed(ext_linegrid)
     startup('--cmd', 'echoerr invalid+')
     screen:expect([[
                                                                   |*4
-      {102:                                                            }|
+      {3:                                                            }|
       {9:Error in pre-vimrc command line:}                            |
       {9:E121: Undefined variable: invalid}                           |
       {6:Press ENTER or type command to continue}^                     |
@@ -74,7 +74,7 @@ local function test_embed(ext_linegrid)
     screen:expect {
       grid = [[
                                                                   |*3
-      {102:                                                            }|
+      {3:                                                            }|
       {9:Error in pre-vimrc command line:}                            |
       {9:foo}                                                         |
       {9:bar}                                                         |