[joiner] require link security for Joiner Entrust (#13046)

This commit adds an explicit check in Joiner::HandleTmf<kUriJoinerEntrust>
to verify that the received message has link-layer security enabled.

According to the Thread specification, the Joiner Entrust message MUST
be protected by link-layer security using the Key Encryption Key (KEK).
Previously, this check was missing, allowing an unauthenticated
attacker to send unsecured Joiner Entrust messages. Such messages
could inject invalid network configuration, causing the device to
fail to attach to the correct network after a reboot.

By verifying IsLinkSecurityEnabled(), we ensure that the message
was successfully decrypted using the KEK (since the network key is
not yet known by the Joiner), thus authenticating the sender as the
valid Commissioner or Joiner Router.

src/core/meshcop/joiner.cpp

@@ -421,7 +421,8 @@ template <> void Joiner::HandleTmf<kUriJoinerEntrust>(Coap::Msg &aMsg)
     Error         error;
     Dataset::Info datasetInfo;
 
-    VerifyOrExit(mState == kStateEntrust && aMsg.IsConfirmable(), error = kErrorDrop);
+    VerifyOrExit(mState == kStateEntrust && aMsg.IsConfirmable() && aMsg.mMessage.IsLinkSecurityEnabled(),
+                 error = kErrorDrop);
 
     LogInfo("Received %s", UriToString<kUriJoinerEntrust>());
     LogCert("[THCI] direction=recv | type=JOIN_ENT.ntf");