[Async] Fix POSIX proactor short write handling in socket sends

writev() can return a short write (fewer bytes than requested) when the
kernel send buffer is nearly full. The POSIX proactor was treating any
positive return as a complete send, dropping unsent bytes. This is
particularly visible on macOS with small SO_SNDBUF values where writev()
routinely returns partial byte counts.

Fix execute_send to detect short writes, advance the iovec past bytes
already consumed, and return WOULD_BLOCK so the operation stays in the
chain for POLLOUT-driven retry. bytes_sent accumulates across retries.

Co-Authored-By: Claude <noreply@anthropic.com>