Fix used_parameters and params types in admin filter stubs

[alessio-b2c2]

Summary

The params argument passed to ListFilter.__init__ and FieldListFilter.__init__ was typed as dict[str, str], but at runtime ChangeList.get_filters builds it from request.GET.lists(), so values are always list[str].

This also means used_parameters — which is populated via prepare_lookup_value() — was incorrectly typed as dict[str, bool | datetime | str]. Since prepare_lookup_value processes all elements in a list through the same key-based branch (__isnull → all bool, otherwise all str), the correct type is dict[str, list[str] | list[bool]].

SimpleListFilter is a special case: it extracts a scalar with value[-1], so its used_parameters is dict[str, str]. This breaks the base class contract and requires a type: ignore[assignment].

While investigating, we also noticed that BooleanFieldListFilter.__init__ checks self.used_parameters[key] in ("1", "0") after super().__init__(). Since values are now list[str] (not scalar str), this comparison is always False — likely a Django bug introduced when filter_params switched from request.GET.items() to request.GET.lists().

Finally, prepare_lookup_value now has proper overloads for list[str] vs str input, and the incorrect datetime was removed from its signature (it never produces datetime values).

Test plan

• All existing tests pass
• Verified runtime behavior by reading Django source (ChangeList.get_filters, FieldListFilter.__init__, SimpleListFilter.__init__, prepare_lookup_value, build_q_object_from_lookup_parameters)
• Confirmed mypy and pyright produce no new errors

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