Fix static_pointer_cast build failure with virtual inheritance in holder_caster_foreign_helpers.h

[rwgk]

When a class uses virtual inheritance and its holder type is shared_ptr, any method taking a shared_ptr of the derived type as an argument fails to compile. This is because set_via_shared_from_this in holder_caster_foreign_helpers.h unconditionally uses static_pointer_cast to downcast from the enable_shared_from_this base to the target type. With virtual inheritance, static_cast from base to derived is ill-formed.

The fix introduces a SFINAE-dispatched esft_downcast helper with two overloads (using the "tag dispatch via overload priority" trick):

• Preferred (int): uses static_pointer_cast — selected when static_cast<type*>(esft_base*) is well-formed (non-virtual inheritance). This is the common case, with zero behavioral change from before.
• Fallback (...): uses dynamic_pointer_cast — selected via SFINAE when the static_cast is ill-formed (virtual inheritance).

Why not simpler alternatives?

• dynamic_pointer_cast unconditionally: requires polymorphic types (at least one virtual function). Would break existing code that uses enable_shared_from_this with non-polymorphic type hierarchies.
• std::is_polymorphic dispatch: orthogonal to virtual inheritance. A type can be polymorphic without virtual inheritance (common case), so this would unnecessarily use dynamic_pointer_cast for all polymorphic types. There is no standard is_virtual_base_of trait.

Note on the test

The test binds .def("name", &SftVirtDerived2::name) directly on the derived class rather than relying on inheritance from SftVirtBase. This works around a separate pre-existing issue where pybind11's method dispatch through virtual inheritance chains uses incorrect pointer offsets, causing a segfault. That issue is unrelated to #5989 and is flagged with a TODO comment in the test code.

CI verification

The first commit (test only, no fix) was pushed to confirm the compilation error reproduces on all CI platforms:

• GCC: cannot convert from pointer to base class 'SftVirtBase' to pointer to derived class 'SftVirtDerived2' via virtual base 'SftVirtDerived'
• MSVC: error C2635: cannot convert a 'SftVirtBase*' to a 'SftVirtDerived2*'; conversion from a virtual base class is implied
• Clang: same class of error

Every platform that compiles tests failed with the same error.

See: https://github.com/pybind/pybind11/actions/runs/23617176855?pr=6014

[rwgk]

Summarizing the CI results for the new test, without a fix:

https://github.com/pybind/pybind11/actions/runs/23617176855?pr=6014

Every platform that compiles the tests fails with the exact same static_pointer_cast error from holder_caster_foreign_helpers.h — exactly as expected.

• GCC: cannot convert from pointer to base class 'SftVirtBase' to pointer to derived class 'SftVirtDerived2' via virtual base 'SftVirtDerived'
• MSVC: error C2635: cannot convert a 'SftVirtBase*' to a 'SftVirtDerived2*'; conversion from a virtual base class is implied