fix(vim.ui)!: change open() to return pcall-like values #28502

Problem:
`vim.ui.open` unnecessarily invents a different success/failure
convention. Its return type was changed in 57adf8c6e0, so we might as
well change it to have a more conventional form.

Solution:
Change the signature to use the `pcall` convention of `status, result`.

runtime/lua/vim/lsp/handlers.lua

@@ -654,15 +654,15 @@ M[ms.window_showDocument] = function(_, result, ctx, _)
 
   if result.external then
     -- TODO(lvimuser): ask the user for confirmation
-    local cmd, err = vim.ui.open(uri)
-    local ret = cmd and cmd:wait(2000) or nil
+    local ok, cmd_or_err = vim.ui.open(uri)
+    local ret = ok and (cmd_or_err --[[@as vim.SystemObj]]):wait(2000) or nil
 
     if ret == nil or ret.code ~= 0 then
       return {
         success = false,
         error = {
           code = protocol.ErrorCodes.UnknownErrorCode,
-          message = ret and ret.stderr or err,
+          message = ret and ret.stderr or cmd_or_err,
         },
       }
     end