[3.11] gh-145986: Avoid unbound C recursion in `conv_content_model` in `pyexpat.c` (CVE 2026-4224) (GH-145987) (#146000)

* [3.11] 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.

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(cherry picked from commit eb0e8be3a7e11b87d198a2c3af1ed0eccf532768)
(cherry picked from commit e5caf45faac)

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

* Update Misc/NEWS.d/next/Security/2026-03-14-17-31-39.gh-issue-145986.ifSSr8.rst

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