Enum list options not displayed in help text

Apologies if this question/bug was already open

**Describe the bug**
The property: `HelpText.AddenumValuesToHelpText` does not work when the option is of type `IEnummerable<{Enum}>`. I'm guessing the list is masking the fact that its containing an enum.

Is it possible to have the same `Valid values: {enum values}` in the help text to be displayed when its a list of enums?

**To Reproduce**
1. Create an option of type `IEnummerable<{Enum}>`
2. configure a parser as follows:
```cs
var parser = new Parser(with =>
{
with.CaseInsensitiveEnumValues = true;
with.HelpWriter = null;
with.AutoVersion = false;
});

var parserResult = parser.ParseArguments(args.SplitArgs());
parserResult
.WithNotParsed(_ =>
{
var helpText = HelpText.AutoBuild(parserResult, h =>
{
h.AutoVersion = false;
h.Heading = string.Empty;
h.Copyright = string.Empty;
h.AddEnumValuesToHelpText = true;

return h;
});
Console.WriteLine(helpText);
});
```
3. use the --help to find no `Valid values: {enum values}` in the help text.

**Expected behavior**
I expected a list of enums to still show available options

**Screenshots**

Start by reproducing the issue with an option typed as IEnumerable, using the Parser and HelpText.AutoBuild configuration shown in the report. Trace how AddEnumValuesToHelpText handles collection option types; done means the generated --help output includes the enum's valid values for list options, while preserving existing behavior for scalar enums.

由索引模型根據 Issue 內容生成。