[php-src] PR #24671: Guard var_export() against native stack overflow (GH-24605)

Pull Request: https://github.com/php/php-src/pull/24671 Author: labishbardiya ## Summary Add native stack overflow protection to var_export() so deeply nested arrays and objects raise a catchable Error instead of risking a segmentation fault. Refs GH-24605. ## Root cause php_var_export_ex() recursively processes nested structures without checking the native stack limit. Other recursive PHP operations already use Zend's stack-limit protection. ## Changes - Add the native stack-limit guard to php_var_export_ex(). - Return FAILURE when the stack limit is exceeded. - Propagate failures from recursive object property exports. - Preserve cleanup on failure paths. - Add a PHPT regression test for deeply nested arrays and objects. ## Validation **Environment:** macOS ARM64, PHP 8.4.28-dev, debug build. - PHP compiled successfully. - GH-24605 regression test: PASS. - Full test suite: 11,549 PASS, 1 FAIL, 3 XFAIL, 9,795 SKIP. The unrelated unexpected failure occurred in php_ini_loaded_file.phpt. It is caused by inherited -c configuration arguments from the test harness. The same test passes when run without those arguments. A correction is already proposed in PHP PR #24406. The full suite has therefore not been demonstrated to pass without unexpected failures under the standard make test invocation. ## Scope This change addresses only the var_export() portion of GH-24605. The print_r() and unserialize() cases remain outside this PR's scope.