[mle] centralize ChildUpdate reject response logic (#12066)

Introduces a new private method `Mle::SendChildUpdateRejectResponse()`
to consolidate the logic for sending a reject response to a
"Child Update Request".

This new method creates a response containing the Source Address TLV,
Status TLV, and (if applicable) Response TLV.

The new method is now used in `Mle::HandleChildUpdateRequestOnChild()`
when the device is not a parent of the sender, and in
`Mle::HandleChildUpdateRequestOnParent()` when a request from an
unknown child is received. This change removes duplicated code from
both locations.

src/core/thread/mle_ftd.cpp

@@ -2255,8 +2255,7 @@ void Mle::HandleChildUpdateRequestOnParent(RxInfo &aRxInfo)
         // Status TLV (error).
         if (mode.IsRxOnWhenIdle())
         {
-            tlvList.Add(Tlv::kStatus);
-            SendChildUpdateResponseToChild(nullptr, aRxInfo.mMessageInfo, tlvList, challenge);
+            IgnoreError(SendChildUpdateRejectResponse(challenge, aRxInfo.mMessageInfo.GetPeerAddr()));
         }
 
         ExitNow();