[mle] check router role allowed before sending multicast adv (#12876)

This commit updates `Mle::SendMulticastAdvertisement()` to verify
that the router role is allowed by calling `IsRouterRoleAllowed()`
before proceeding to send the multicast MLE advertisement.

src/core/thread/mle_ftd.cpp

@@ -539,7 +539,14 @@ exit:
     return;
 }
 
-void Mle::SendMulticastAdvertisement(void) { SendAdvertisement(Ip6::Address::GetLinkLocalAllNodesMulticast()); }
+void Mle::SendMulticastAdvertisement(void)
+{
+    VerifyOrExit(IsRouterRoleAllowed());
+    SendAdvertisement(Ip6::Address::GetLinkLocalAllNodesMulticast());
+
+exit:
+    return;
+}
 
 void Mle::ScheduleUnicastAdvertisementTo(const Router &aRouter)
 {