[php-src] PR #25387: Fix uploaded files leak in request_parse_body()

Pull Request: https://github.com/php/php-src/pull/25387 Author: dunglas While working on the "[Request lifecycle hooks for SAPIs with a worker mode](https://wiki.php.net/rfc/request_lifecycle_hooks)" RFC, I discovered a bug affecting existing SAPIs. Calling request_parse_body() on a multipart request whose body was already parsed replaces SG(rfc1867_uploaded_files) with a new instance. This happens when the body was parsed at startup (POST with enable_post_data_reading=1), or when the userland request_parse_body() function is called several times. The files from the previous parses are then lost. This patch reuses the existing table instead. Related to #24802 and #24987, but neither the same bug nor the same fix. AI: Claude Code used to find the issue and write the patch.