refactor(detach): eliminate nvim__ui_detach #40793

Problem:
`nvim__ui_detach` was added in 85e0559d46 in order to implement
`detach_others` from Lua. Using Lua in this case is doing more harm than
good.

Solution:
Extract `ui_detach_channel`, which also allows it to be used for the
"self detach" path from `ex_detach`.

Bonus: the old MSWIN path always called `os_swap_to_hidden_console()`;
now `ui_detach_channel` only does so for a stdio channel. For the common
stdio TUI this is identical; for a socket UI it's more correct (no
parent console to swap).

runtime/lua/vim/_meta/api.gen.lua

@@ -186,16 +186,6 @@ function vim.api.nvim__set_restart_on_crash(progpath, argv) end
 --- @return table<string,any> # Map of various internal stats.
 function vim.api.nvim__stats() end
 
---- WARNING: This feature is experimental/unstable.
----
---- Detaches the UI on channel `chan`.
----
---- Sets the channel's detach flag (so the server doesn't exit on stdio UIs),
---- sends an "error_exit" UI event, and closes the channel.
----
---- @param chan integer UI channel to disconnect.
-function vim.api.nvim__ui_detach(chan) end
-
 --- WARNING: This feature is experimental/unstable.
 ---
 --- @param str string