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 b299f32104.

* Disable test_shared_ptr_return_for_unique_ptr_holder when PYBIND11_TEST_SMART_HOLDER=ON

* Add counterexample

---------

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>

tests/pybind11_tests.cpp

@@ -96,6 +96,12 @@ PYBIND11_MODULE(pybind11_tests, m, py::mod_gil_not_used()) {
 #else
         false;
 #endif
+    m.attr("PYBIND11_TEST_SMART_HOLDER") =
+#if defined(PYBIND11_RUN_TESTING_WITH_SMART_HOLDER_AS_DEFAULT_BUT_NEVER_USE_IN_PRODUCTION_PLEASE)
+        true;
+#else
+        false;
+#endif
 
     bind_ConstructorStats(m);