Secure channel: seal symmetric chunks in place

SealSymmetricMessage copied the encoded chunks into a second
BufferCollection. The chunks are now replaced in place by the segments
to send, which are over the same buffers, so the encoded collection is
the one sent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Opc.Ua.Core/Stack/Tcp/UaSCBinaryChannel.Symmetric.cs

@@ -939,7 +939,8 @@ namespace Opc.Ua.Bindings
                 const int headerSize = TcpMessageLimits.SymmetricHeaderSize +
                     TcpMessageLimits.SequenceHeaderSize;
 
-                var chunksToSend = new BufferCollection(chunksToProcess.Count);
+                // the chunks are secured in place: each body segment is replaced by the
+                // segment to send, which covers the headers in the same buffer.
                 int lastChunk = chunksToProcess.Count - 1;
 
                 for (int ii = 0; ii < chunksToProcess.Count; ii++)
@@ -1028,13 +1029,12 @@ namespace Opc.Ua.Bindings
                             position);
                     }
 
-                    // add the header into chunk.
-                    chunksToSend.Add(dataToSend);
+                    chunksToProcess[ii] = dataToSend;
                 }
 
                 // ensure the buffers don't get cleaned up on exit.
                 success = true;
-                return chunksToSend;
+                return chunksToProcess;
             }
             finally
             {