fix(smart_holder): keep void-cast semantics in from_unique_ptr

[henryiii]

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smart_holder::from_unique_ptr() always built the owning shared_ptr from a T *. This connects the std::enable_shared_from_this<T> machinery to the holder control block. For trampoline (alias) types this must not happen: shared_from_this() must fail with bad_weak_ptr, because the resulting shared_ptr does not keep the Python object alive.

The void_cast_raw_ptr flag in detail/init.h was made ineffective by #5836, which changed the second parameter of from_unique_ptr() into a multiple-inheritance subobject pointer. As a result, a factory that returns a unique_ptr to a trampoline lost the guard, and the Python override could disappear silently.

from_unique_ptr() now has an explicit void_cast_raw_ptr parameter. If it is true, the owner is constructed as a shared_ptr<void>, which keeps the control block invisible to shared_from_this(). The multiple-inheritance behavior does not change.

Fixes item 4 of #6159.

Suggested changelog entry:

• Fixed py::init() factories that return a std::unique_ptr to a trampoline: shared_from_this() correctly fails with bad_weak_ptr again, instead of returning a shared_ptr that does not keep the Python object alive.