[mle] configure poll/rx-on modes after child role change (#7101)

If we configure CSL parameters before a command to attach, then the
child has configured valid CSL state, but when trying to configure the
auto-sync CSL poll timeout, the check to IsCslEnabled->IsCslCapable
fails because IsChild() fails.

When handling the child ID response success, we now trigger
poll-sender's recalculation, and rx-on-idle states, after role change
to child.

src/core/thread/mle.cpp

@@ -3681,16 +3681,6 @@ void Mle::HandleChildIdResponse(const Message &         aMessage,
 
     SetLeaderData(leaderData.GetPartitionId(), leaderData.GetWeighting(), leaderData.GetLeaderRouterId());
 
-    if (!IsRxOnWhenIdle())
-    {
-        Get<DataPollSender>().SetAttachMode(false);
-        Get<MeshForwarder>().SetRxOnWhenIdle(false);
-    }
-    else
-    {
-        Get<MeshForwarder>().SetRxOnWhenIdle(true);
-    }
-
 #if OPENTHREAD_FTD
     if (IsFullThreadDevice())
     {
@@ -3719,6 +3709,16 @@ void Mle::HandleChildIdResponse(const Message &         aMessage,
 
     SetStateChild(shortAddress);
 
+    if (!IsRxOnWhenIdle())
+    {
+        Get<DataPollSender>().SetAttachMode(false);
+        Get<MeshForwarder>().SetRxOnWhenIdle(false);
+    }
+    else
+    {
+        Get<MeshForwarder>().SetRxOnWhenIdle(true);
+    }
+
 exit:
     LogProcessError(kTypeChildIdResponse, error);
 }