[mle] echo back "Supervision TLV" in Child ID Response (#9061)

This commit updates the `MleRouter` to echo back the "Supervision
TLV" in MLE Child ID Response message when it is included in
MLE Child ID Request by a child.

src/core/thread/mle_router.cpp

@@ -2003,6 +2003,7 @@ void MleRouter::HandleChildIdRequest(RxInfo &aRxInfo)
     MeshCoP::Timestamp timestamp;
     bool               needsActiveDatasetTlv;
     bool               needsPendingDatasetTlv;
+    bool               needsSupervisionTlv;
     Child             *child;
     Router            *router;
     uint8_t            numTlvs;
@@ -2046,9 +2047,11 @@ void MleRouter::HandleChildIdRequest(RxInfo &aRxInfo)
     SuccessOrExit(error = Tlv::Find<TimeoutTlv>(aRxInfo.mMessage, timeout));
 
     // Supervision interval
+    needsSupervisionTlv = false;
     switch (Tlv::Find<SupervisionIntervalTlv>(aRxInfo.mMessage, supervisionInterval))
     {
     case kErrorNone:
+        needsSupervisionTlv = true;
         break;
     case kErrorNotFound:
         supervisionInterval = (version <= kThreadVersion1p3) ? kChildSupervisionDefaultIntervalForOlderVersion : 0;
@@ -2100,6 +2103,11 @@ void MleRouter::HandleChildIdRequest(RxInfo &aRxInfo)
         numTlvs++;
     }
 
+    if (needsSupervisionTlv)
+    {
+        numTlvs++;
+    }
+
     VerifyOrExit(numTlvs <= Child::kMaxRequestTlvs, error = kErrorParse);
 
     if (!mode.IsFullThreadDevice())
@@ -2157,6 +2165,11 @@ void MleRouter::HandleChildIdRequest(RxInfo &aRxInfo)
         child->SetRequestTlv(numTlvs++, Tlv::kPendingDataset);
     }
 
+    if (needsSupervisionTlv)
+    {
+        child->SetRequestTlv(numTlvs++, Tlv::kSupervisionInterval);
+    }
+
     aRxInfo.mClass = RxInfo::kAuthoritativeMessage;
 
     switch (mRole)
@@ -2924,6 +2937,10 @@ Error MleRouter::SendChildIdResponse(Child &aChild)
             SuccessOrExit(error = message->AppendPendingDatasetTlv());
             break;
 
+        case Tlv::kSupervisionInterval:
+            SuccessOrExit(error = message->AppendSupervisionIntervalTlv(aChild.GetSupervisionInterval()));
+            break;
+
         default:
             break;
         }

src/core/thread/topology.hpp

@@ -873,7 +873,7 @@ class Child : public Neighbor,
     class AddressIteratorBuilder;
 
 public:
-    static constexpr uint8_t kMaxRequestTlvs = 5;
+    static constexpr uint8_t kMaxRequestTlvs = 6;
 
     /**
      * This class represents diagnostic information for a Thread Child.