[php-src] PR #24688: Test precision cap in zend_gcvt() for GH-24563

Pull Request: https://github.com/php/php-src/pull/24688 Author: aasul this fixes the excessive temporary allocations caused by unusually large serialize_precision values during float to string conversion currently zend_gcvt() passes ndigit directly to zend_dtoa(), which can cause multi MB Bigint allocations even when the actual formatted output is much shorter the patch caps positive precision values at 770 before calling zend_dtoa(). this is based on the maximum 767 significant decimal digits needed to represent a binary64 value exactly. negative precision and the existing default behavior are left unchanged the original problem can be reproduced with: ```php ini_set('serialize_precision', 4194304); echo var_export(0.1, true); ``` under tight RLIMIT_AS limits this can trigger Balloc() failed to allocate memory on the unpatched build i tested the change on Linux x86_64 using NTS and ZTS debug builds. with the patch, the largest conversion malloc dropped from 4194332 to 1052 bytes and peak tracked live allocation dropped from 4194492 to 1212 bytes. peak virtual memory also decreased by roughly 4 MB i compared the output across 387 test value entries including subnormals, large doubles, negative zero, INF, NAN and different precision settings. no byte for byte differences were found between the original and patched builds FPM testing covered 648 requests under controlled memory limits. the baseline produced 20 Balloc() fatals per worker while the patched version completed those conversions under the same conditions. subsequent requests continued working correctly and the worker PIDs remained unchanged the relevant NTS and ZTS tests passed with 412 passes, 43 skips and no failures each, including the new PHPT @srayuth089 also independently tested the patch on macOS arm64 and confirmed that the original failure no longer occurs within the tested memory limits. their 302 conversion battery produced identical output between the original and patched builds, including with serialize_precision=4194304 this change only addresses the excessive precision allocation path through zend_gcvt(). it doesn't change the general OOM handling or other direct zend_dtoa() callers thanks to @srayuth089 for suggesting the common zend_gcvt() cap and independently verifying the patch related to #24563 > the investigation was assisted by AI tools and reviewed by me