fix: handle null `py::handle` and add tests for `py::scoped_critical_section` (#5706)

* chore: handle null for `py::scoped_critical_section`

* test: add tests for `py::scoped_critical_section`

* test: use assert instead of REQUIRE

* feat: enable faulthandler for pytest

* chore: use `__has_include(<barrier>)`

* fix: fix segmentation fault in test

* fix: test critical_section for no-gil only

* test: run new tests only

* test: ensure non-empty test selection

* fix: fix test critical_section

* fix: change Python 3.14.0b1/2 xfail tests to non-strict

* test: trigger gc manually

* test: mark xfail to `DynamicClass`

* Use `namespace test_scoped_critical_section_ns` (standard approach to guard against name clashes).

* Simplify changes in pybind11/critical_section.h and add test_nullptr_combinations()

* test: disable Python devmode in pytest

* test: add comprehensive comments for the tests

* test: add a summary comment for tests

* refactor: simpler impl

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

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Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>
Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>