fix(system): use native path for Windows executables #40885

Problem:
When 'shellslash' is set, exepath() returns forward-slashed paths.
Passing that path to vim.system() can prevent cmd.exe from launching.

Solution:
Convert the resolved executable path to native separators before spawning.
Add a Windows regression test for cmd.exe with 'shellslash' enabled.

runtime/lua/vim/_core/system.lua

@@ -319,7 +319,8 @@ local function spawn(cmd, opts, on_exit, on_error)
   if is_win then
     local cmd1 = vim.fn.exepath(cmd)
     if cmd1 ~= '' then
-      cmd = cmd1
+      -- Use native separators when launching a resolved Windows executable.
+      cmd = cmd1:gsub('/', '\\')
     end
   end