[php-src] PR #24618: Fix GH-24492: handle base64_encode() soft OOM

Pull Request: https://github.com/php/php-src/pull/24618 Author: forgeblueai ## Problem base64_encode() currently uses the non-fallible php_base64_encode_ex() allocation contract. A userland call requiring a large output can hit PHP's hard memory limit, terminate with exit 137, and skip the registered shutdown handler. This is part of the previously reported [GH-24492](https://github.com/php/php-src/issues/24492) behavior. ## Fix Add an internal PHP_BASE64_FALLIBLE option used **only** by the PHP base64_encode() entry point. Make the five generic and x86 SIMD encoding allocation sites honor that option through zend_string_safe_falloc(); return NULL before dereferencing the failed allocation. The PHP entry point calls RETURN_OOM(). Native C helpers called without the new flag (including existing callers using PHP_BASE64_NO_PADDING) preserve their non-fallible behavior, avoiding a breaking change to the exported helper contract. ## Reproduction and tests The accompanying ext/standard/tests/url/gh24492_base64_encode_oom.phpt confirms that the input is constructed before invoking Base64, that memory exhaustion is reported through the soft memory-limit fatal path, and that shutdown handlers run. Verified experiments from [ForgeBlue PHP Core Lab](https://github.com/forgeblueai/php-core-lab): - [Pinned PHP 8.4.27 and 8.5.12](https://github.com/forgeblueai/php-core-lab/actions/runs/39996126737): before patch exit 137 without shutdown, after patch exit 255 with soft-OOM message and shutdown. - [ZTS/extension caller tests](https://github.com/forgeblueai/php-core-lab/actions/runs/39999223589): 9/9 focused PHPTs on both 8.4.27 and 8.5.12, including representative OpenSSL, iconv and SOAP consumers. - [Current PHP-8.4 HEAD NTS and ZTS](https://github.com/forgeblueai/php-core-lab/actions/runs/40000337560): 9/9 focused PHPTs in each configuration, with before/after OOM verification, based on source SHA fc05c9bf2486d3d66593501bf78c1d83c32ecbc5. - [Patched 8.5.12 sanitizer baseline](https://github.com/forgeblueai/php-core-lab/actions/runs/39996126737): targeted ASan/UBSan tests passed; this is not full memory-safety validation. This is deliberately a **partial fix**. Existing [#24548](https://github.com/php/php-src/pull/24548) and [#24596](https://github.com/php/php-src/pull/24596) address other allocation paths. ## Limits and review notes Broader PHP test suites, all CPU-selected SIMD implementations, and ARM64-specific validation remain appropriate follow-up work. This patch does not claim a new security vulnerability or CVE.