[php-src] PR #24638: Add FPM regression test for fatal backtrace reset

Pull Request: https://github.com/php/php-src/pull/24638 Author: aasul added an FPM regression test for the fatal backtrace cleanup from CVE-2026-109619 the test triggers a fatal error then sends another request to the same FPM worker with an invalid timezone setting. it checks that the previous backtrace doesnt carry over into the next request's warning and that the worker still works normally tested on PHP 8.5.13-dev NTS debug and it passed. also tested against the pre-fix commit dd830558ffb with ASan and fast shutdown which failed twice with the expected memory error. the same test passed on the patched 31c0ea7d with no sanitizer errors only added a PHPT test, no changes to the C code related to https://github.com/php/php-src/security/advisories/GHSA-6wmf-4rv5-p3rp