[3.13] gh-145986: Avoid unbound C recursion in `conv_content_model` in `pyexpat.c` (CVE 2026-4224) (GH-145987) (#145996)

* gh-145986: Avoid unbound C recursion in `conv_content_model` in `pyexpat.c` (CVE 2026-4224) (GH-145987)

Fix C stack overflow (CVE-2026-4224) when an Expat parser
with a registered `ElementDeclHandler` parses inline DTD
containing deeply nested content model.

---------
(cherry picked from commit eb0e8be3a7e11b87d198a2c3af1ed0eccf532768)

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

* Remvoe `skip_if_unlimited_stack_size` decorator

* Remove more decorators not on this branch

---------

Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>