fix: serialize typed values in ChoiceListParameter

[winklemad]

Description

ChoiceListParameter.serialize joins the raw values:

def serialize(self, x):
    return self._sep.join(x)

But str.join requires every element to be a str, while ChoiceListParameter.parse produces values of self._var_type (return self.normalize(map(self._var_type, values))). So with var_type=int (or float), serialize raises TypeError, and the parse/serialize round trip is broken by construction.

Because serialize is called from Task.to_str_params during task_id computation, an int/float ChoiceListParameter is completely unusable — instantiating any task that uses one throws:

class Foo(luigi.Task):
    args = luigi.ChoiceListParameter(choices=[1, 2, 3], var_type=int)

Foo(args=(1, 3))
# TypeError: sequence item 0: expected str instance, int found

int choices are a documented, typed use case (test/mypy_test.py), and the sibling ChoiceParameter (via the default str(x) serializer) and EnumListParameter (self._sep.join([e.name for e in x])) both handle non-str values correctly — only ChoiceListParameter joins raw typed values.

Fix

Serialize each value with str(), matching EnumListParameter:

return self._sep.join(str(v) for v in x)

No change for the existing var_type=str case (str("1") == "1"); it restores the parse/serialize round trip for int/float.

Testing

Added test_choice_list_param_typed_serialize_parse, asserting int and float ChoiceListParameter serialize and round-trip correctly, with the str case unchanged. It fails before the change (TypeError) and passes after; test/parameter_test.py passes and ruff check / ruff format --check are clean.