[php-src] PR #23439: Fix read buffer compaction in stream filter flush

Pull Request: https://github.com/php/php-src/pull/23439 Author: crystarm php_stream_filter_flush() compacts unread data before appending buckets produced by a read filter. The source and destination ranges may overlap, making the use of memcpy() undefined behavior. Additionally, readpos was reset before it was subtracted from writepos, so the buffer size was not adjusted and stale data could remain visible. Use memmove() and adjust writepos before resetting readpos, matching the existing buffer compaction logic in php_stream_fill_read_buffer(). The issue was detected by static analysis: [BUFFER_OVERLAP filter.c:[458:4].log](https://github.com/user-attachments/files/31381411/BUFFER_OVERLAP.filter.c.458.4.log)