fix(window): don't leak fold memory with style=minimal #38287

Problem: discarding saved option values in the WinInfo of closed style=minimal
windows leaks memory.

Solution: also free the nested folds.

test/functional/api/window_spec.lua

@@ -3650,5 +3650,11 @@ describe('API/win', function()
       eq(1, eval('&number'))
       eq(1, eval('&rightleft')) -- unrelated option unaffected
     end)
+
+    it('minimal style does not leak WinInfo fold memory', function()
+      feed('zfG')
+      api.nvim_open_win(0, true, { split = 'below', style = 'minimal' })
+      command('quit')
+    end)
   end)
 end)