fix(lua): avoid __index when deciding if a table is a list #39556

Problem:
When a table has `__index`, `vim.islist` is unreliable.

Solution:
Index using `rawget`.

runtime/lua/vim/_core/shared.lua

@@ -960,7 +960,7 @@ function vim.islist(t)
   for _ in
     pairs(t--[[@as table<any,any>]])
   do
-    if t[j] == nil then
+    if rawget(t, j) == nil then
       return false
     end
     j = j + 1

test/functional/lua/vim_spec.lua

@@ -969,6 +969,17 @@ describe('lua stdlib', function()
     eq(false, exec_lua('return vim.islist({1, 2, nil, 4})'))
     eq(false, exec_lua('return vim.islist({nil, 2, 3, 4})'))
     eq(false, exec_lua('return vim.islist({1, [1.5]=2, [3]=3})'))
+    eq(
+      false,
+      exec_lua([[
+        local t = setmetatable({ 1, [3] = 3 }, {
+          __index = function()
+            return 2
+          end,
+        })
+        return vim.islist(t)
+      ]])
+    )
   end)
 
   it('vim.tbl_isempty', function()