experimental-inspect: give slot-wrapper trailing arguments their None default

[jonasdedden]

What was wrong

PyMethod::parse marks a protocol method's parameters positional-only but never records that CPython's slot wrappers substitute None for an omitted trailing argument. So __pow__(&self, other, modulo: Option<&Self>) was introspected as def __pow__(self, other: object, modulo: object, /) while the runtime wrapper advertises ($self, value, mod=None, /). Same defect for __get__, whose wrapper advertises ($self, instance, owner=None, /).

The fix

The fact lives on the slot definition, next to the arity it belongs with, rather than in a name match. SlotDef and SlotFragmentDef gain an optional_trailing_args: usize, set through the existing const builder chain:

    const __GET__: SlotDef = SlotDef::new("Py_tp_descr_get", "descrgetfunc")
        // `__get__($self, instance, owner=None, /)`
        .with_optional_trailing_args(1);

PyMethodProtoKind::optional_trailing_args delegates to whichever definition applies, and PyMethod::parse passes the count to FunctionSignature::default_trailing_parameters_to_none(count) beside the existing make_all_parameters_positional_only call. Codegen destructuring sites bind the new field as _, so it stays introspection-only and a future field still forces a decision.

__rpow__ is included because its wrapper carries the same mod=None.

Why a plain count, and why always None

The three affected methods are not an arbitrary selection. Scanning every builtin type for slot wrappers whose __text_signature__ carries a default yields exactly three, and all three default to None:

    __get__    ($self, instance, owner=None, /)
    __pow__    ($self, value, mod=None, /)
    __rpow__   ($self, value, mod=None, /)

A non-None sentinel cannot arise. An omitted argument reaches a slot wrapper as NULL at the C level and the wrapper substitutes None. There is no channel for any other default, so a count is sufficient and not lossy. If Python ever adds a slot wrapper with a different default, the field becomes &'static [Option<&'static str>] and only the three call sites change.

Left out

__ipow__. PyO3 gives it a third parameter, but CPython's IBSLOT entry advertises ($self, value, /) and slot_nb_inplace_power drops the modulo, so a None default there would not match the runtime. Confirmed the IBSLOT shape against list.__iadd__.__text_signature__.