fix(cmdwin): set 'buflisted'

Problem:
Legacy cmdwin set 'buflisted', but new one doesn't.
https://github.com/neovim/neovim/issues/40431#issuecomment-4811593353

'buflisted' useful for:
- "bufferline" style tablines, they usually show only listed buffers.
- some automatic actions can be conditioned on whether a buffer is
  listed; 'buflisted' signals that the buffer is "important enough".

Solution:
Set 'buflisted'.

runtime/lua/vim/_core/cmdwin.lua

@@ -74,7 +74,7 @@ function M.open(type, init_line, init_col)
   vim.bo[buf].buftype = 'nofile'
   vim.bo[buf].bufhidden = 'wipe'
   vim.bo[buf].swapfile = false
-  vim.bo[buf].buflisted = false
+  vim.bo[buf].buflisted = true -- #40431
   vim.wo[win][0].winfixbuf = true
   vim.wo[win][0].foldenable = false
   -- Show cmdwin-char via 'statuscolumn'.

test/functional/editor/cmdwin_spec.lua

@@ -22,6 +22,7 @@ describe('cmdwin', function()
     eq('nofile', api.nvim_get_option_value('buftype', { buf = 0 }))
     eq('wipe', api.nvim_get_option_value('bufhidden', { buf = 0 }))
     eq(false, api.nvim_get_option_value('swapfile', { buf = 0 }))
+    eq(true, api.nvim_get_option_value('buflisted', { buf = 0 })) -- #40431
     eq(true, api.nvim_get_option_value('winfixbuf', { win = 0 }))
 
     -- <CR> executes the cmdline