Fix `static_pointer_cast` build failure with virtual inheritance in `holder_caster_foreign_helpers.h` (#6014)

* Add regression test for #5989: static_pointer_cast fails with virtual inheritance

When a class uses virtual inheritance and its holder type is shared_ptr,
passing a shared_ptr of the derived type as a method argument triggers
a compilation error because static_pointer_cast cannot downcast through
a virtual base (dynamic_pointer_cast is needed instead).

Made-with: Cursor

* Fix #5989: use dynamic_pointer_cast for virtual inheritance in esft downcast

Replace the unconditional static_pointer_cast in set_via_shared_from_this
with a SFINAE-dispatched esft_downcast helper that falls back to
dynamic_pointer_cast when static_cast through a virtual base is ill-formed.

Also add a workaround in the test binding (.def("name") on SftVirtDerived2)
for a separate pre-existing issue with inherited method dispatch through
virtual bases.

Made-with: Cursor