[meshcop-leader] check IsLeader() processing `LeaderPetition/KeepAlive (#9531)

This commit adds an `IsLeader()` check to ensure that the device is
acting as the leader before processing `kUriLeaderPetition` or
`kUriLeaderKeepAlive` TMF messages. This adds a safeguard to ensure
that only the leader can update the commissioner dataset.

src/core/meshcop/meshcop_leader.cpp

@@ -73,6 +73,8 @@ template <> void Leader::HandleTmf<kUriLeaderPetition>(Coap::Message &aMessage,
 
     LogInfo("Received %s", UriToString<kUriLeaderPetition>());
 
+    VerifyOrExit(Get<Mle::MleRouter>().IsLeader());
+
     VerifyOrExit(Get<Mle::MleRouter>().IsRoutingLocator(aMessageInfo.GetPeerAddr()));
     SuccessOrExit(Tlv::FindTlv(aMessage, commissionerId));
 
@@ -152,6 +154,8 @@ template <> void Leader::HandleTmf<kUriLeaderKeepAlive>(Coap::Message &aMessage,
 
     LogInfo("Received %s", UriToString<kUriLeaderKeepAlive>());
 
+    VerifyOrExit(Get<Mle::MleRouter>().IsLeader());
+
     SuccessOrExit(Tlv::Find<StateTlv>(aMessage, state));
 
     SuccessOrExit(Tlv::Find<CommissionerSessionIdTlv>(aMessage, sessionId));