fix truncation check: use == instead of >= for buffer-full detection

recv_from can never return more bytes than the buffer size — the kernel
truncates silently. == is the correct heuristic for detecting truncation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/forward.rs

@@ -23,7 +23,7 @@ pub async fn forward_query(
     let mut recv_buffer = BytePacketBuffer::new();
     let (size, _) = timeout(timeout_duration, socket.recv_from(&mut recv_buffer.buf)).await??;
 
-    if size >= recv_buffer.buf.len() {
+    if size == recv_buffer.buf.len() {
         log::debug!(
             "upstream response truncated ({} bytes, buffer {})",
             size,