[coap] block2 fix PrepareNextBlockRequest to provide token from next request (#12097)

src/core/coap/coap.cpp

@@ -640,6 +640,8 @@ Error CoapBase::PrepareNextBlockRequest(Message::BlockType aType,
     blockOption = (aType == Message::kBlockType1) ? kOptionBlock1 : kOptionBlock2;
 
     aRequest.Init(kTypeConfirmable, static_cast<ot::Coap::Code>(aRequestOld.GetCode()));
+    // Per RFC 7959, all requests in a block-wise transfer MUST use the same token.
+    IgnoreError(aRequest.SetTokenFromMessage(aRequestOld));
     SuccessOrExit(error = iterator.Init(aRequestOld));
 
     // Copy options from last response to next message