fix(generator): treat [ArgumentParser] on collection property as whole-collection parser

[Mpdreamz]

Summary

• When [ArgumentParser(typeof(P))] is placed on an IReadOnlySet<T> (or any collection-typed) property/parameter, the generator was routing it through BuildCollectionParameterModel and calling P.TryParse per element, then trying to Add the IReadOnlySet<T> result into a HashSet<T> — a type mismatch when P implements IArgumentParser<IReadOnlySet<T>>.
• Fix: guard all five TryUnwrapCollectionType call sites with TryParserTypeFqFromSymbol(symbol) is null. When [ArgumentParser] is present, the property falls through to the scalar ClassifyScalarUnified path, which classifies it as CliScalarKind.CustomParser with CustomValueTypeFq equal to the full collection type.
• The emitter then generates a single flags.TryGetValue lookup and a direct parser.TryParse assignment — the correct behavior for a whole-collection parser.

Generated code before (broken)

foreach (var __raw in __rawList_options__exporters)
{
    var __parser = new ExporterParser();
    __parser.TryParse(__raw!, out var __pv);        // __pv is IReadOnlySet<Exporter>
    options__exporters_acc.Add(__ce_options__exporters); // acc is HashSet<Exporter> — type mismatch
}

Generated code after (correct)

if (flags.TryGetValue("exporters", out var options__exportersText))
{
    var __parser = new ExporterParser();
    if (!__parser.TryParse(options__exportersText!, out var __pv))
    {
        Console.Error.WriteLine("Error: invalid value for --exporters.");
        return 2;
    }
    options__exporters = __pv;
}

Test plan

• WholeCollection_ArgumentParser_on_IReadOnlySet_parameter — required flag, comma-separated value parsed by StringSetParser : IArgumentParser<IReadOnlySet<string>>
• WholeCollection_ArgumentParser_on_nullable_IReadOnlySet_parameter_when_provided — optional, value present
• WholeCollection_ArgumentParser_on_nullable_IReadOnlySet_parameter_when_absent — optional, flag omitted → null
• Full suite: 173 integration + 91 unit tests all pass

🤖 Generated with Claude Code