Fix crash in def_readwrite for non-smart-holder properties of smart-holder classes (v2) (#6008)

* Add tests that cause crash in def_readwrite

- Occurs with non-smart-holder property of smart-holder class

* Fix crash in def_readwrite for non-smart-holder properties of smart-holder classes

* Use default policy

* Address PR comments

* Add test for cast error path

* style: pre-commit fixes

* Revert "Use default policy"

This reverts commit b299f321044ff946701a0020a4549f3eb0525b83.

* Disable test_shared_ptr_return_for_unique_ptr_holder when PYBIND11_TEST_SMART_HOLDER=ON

* Add counterexample

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>