fix(loader): remove all cached loaders on disable #41613

Problem:
When disabling, the loader list is traversed forwards while entries are
removed. As a result, some entries are skipped and remain active. Thus,
disabling `vim.loader` restores `_G.loadfile`, but does not fully
restore the original package loader chain.

Solution:
Traverse `package.loaders` in reverse when removing cached loaders.
Capture the original `package.loaders` list in the test, and assert that
the list is restored properly.

runtime/lua/vim/loader.lua

@@ -452,7 +452,8 @@ function M.enable(enable)
     end
   else
     _G.loadfile = _loadfile
-    for l, loader in ipairs(loaders) do
+    for l = #loaders, 1, -1 do
+      local loader = loaders[l]
       if loader == loader_cached or loader == loader_lib_cached then
         table.remove(loaders, l)
       end