[php-src] PR #23969: sapi/cli: Fix built-in server truncating responses after a partial write

Pull Request: https://github.com/php/php-src/pull/23969 Author: iliaal After a short send() of one chunk, php_cli_server_content_sender_send() went on to the next one. If that send succeeded, its bytes went out ahead of the remainder and the full-send branch unlinked the partially sent chunk, so multi-chunk responses such as error pages were cut short, the connection closed before Content-Length was reached, and the remainder leaked. It now stops at the first partial write and resumes from that chunk on the next POLLOUT. There is no test, since triggering it depends on TCP timing.