[skip ci] test: reject shared_ptr casts for custom holders (#6069)

Lock down the current behavior discussed in issue #6064: py::cast() from std::shared_ptr<T> remains rejected for custom holder bindings that are not std::shared_ptr or py::smart_holder.

This extracts the incompatible-holder coverage from PR #6068 and adds a test shaped like issue #6064, while PR #6065 and PR #6066 explore alternative support paths.