[nexus] fix IPv6 receive callback setup in node reset (#12845)

This commit updates `Node::Reset()` in the Nexus platform to correctly
set the IPv6 receive callback after the OpenThread `Instance` has been
re-initialized via placement `new`.

Previously, `otIp6SetReceiveCallback()` was called before the new
`Instance` was constructed, meaning the callback registration would
be lost when the instance memory was overwritten. Additionally, the
callback registration now passes the associated `Node` object as the
context.

tests/nexus/platform/nexus_node.cpp

@@ -43,7 +43,6 @@ void Node::Reset(void)
     mInfraIf.mPendingTxQueue.DequeueAndFreeAll();
     mPendingTasklet = false;
 
-    otIp6SetReceiveCallback(&GetInstance(), HandleIp6Receive, &GetInstance());
 #if OPENTHREAD_CONFIG_RADIO_LINK_TREL_ENABLE
     mTrel.Reset();
 #endif
@@ -53,6 +52,8 @@ void Node::Reset(void)
     instance = new (instance) Instance();
     instance->SetId(id);
     instance->AfterInit();
+
+    otIp6SetReceiveCallback(instance, Node::HandleIp6Receive, this);
 }
 
 void Node::Form(void)