fix(vim.ui): open() may wait indefinitely #28325

Problem:
vim.ui.open "locks up" Nvim if the spawned process does not terminate. #27986

Solution:
- Change `vim.ui.open()`:
    - Do not call `wait()`.
    - Return a `SystemObj`. The caller can decide if it wants to `wait()`.
- Change `gx` to `wait()` only a short time.
    - Allows `gx` to show a message if the command fails, without the
      risk of waiting forever.

runtime/lua/vim/_defaults.lua

@@ -95,10 +95,19 @@ do
     { silent = true, expr = true, desc = ':help v_@-default' }
   )
 
-  --- Map |gx| to call |vim.ui.open| on the identifier under the cursor
+  --- Map |gx| to call |vim.ui.open| on the <cfile> at cursor.
   do
     local function do_open(uri)
-      local _, err = vim.ui.open(uri)
+      local cmd, err = vim.ui.open(uri)
+      local rv = cmd and cmd:wait(1000) or nil
+      if cmd and rv and rv.code ~= 0 then
+        err = ('vim.ui.open: command %s (%d): %s'):format(
+          (rv.code == 124 and 'timeout' or 'failed'),
+          rv.code,
+          vim.inspect(cmd.cmd)
+        )
+      end
+
       if err then
         vim.notify(err, vim.log.levels.ERROR)
       end