fix(smart_holder): keep void-cast semantics in from_unique_ptr (#6163) * fix(smart_holder): keep void-cast semantics in from_unique_ptr Fixes item 4 of #6159. * test: cover base unique_ptr factories and trampoline lifetime * test: limit prompt trampoline destruction check to CPython * docs: clarify smart_holder void-cast ownership safeguard --------- Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>