gh-145986: Avoid unbound C recursion in `conv_content_model` in `pyexpat.c` (CVE 2026-4224) (#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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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>