[mesh-forwarder] evict insecure reassembly messages first (#13211)

Enhance `MeshForwarder::EvictMessage` to prioritize evicting messages
from `mReassemblyList` that were received without link security when
reclaiming message buffers (reason `kEvictReasonNoMessageBuffer`).

This helps protect secure messages in the send queue from being
evicted due to buffer exhaustion, by prioritizing the dropping of
insecure, potentially incomplete, reassembled fragments.

Both FTD and MTD implementations of `EvictMessage` are updated.

src/core/thread/mesh_forwarder.cpp

@@ -1201,6 +1201,27 @@ void MeshForwarder::ClearReassemblyList(void)
     }
 }
 
+Error MeshForwarder::RemoveUnsecureReassemblyMessage(EvictReason aEvictReason)
+{
+    Error error = kErrorNotFound;
+
+    VerifyOrExit(aEvictReason == kEvictReasonNoMessageBuffer);
+
+    for (Message &message : mReassemblyList)
+    {
+        if (!message.IsLinkSecurityEnabled())
+        {
+            LogMessage(kMessageReassemblyDrop, message, kErrorNoBufs);
+            mCounters.UpdateOnDrop(message);
+            mReassemblyList.DequeueAndFree(message);
+            ExitNow(error = kErrorNone);
+        }
+    }
+
+exit:
+    return error;
+}
+
 void MeshForwarder::HandleTimeTick(void)
 {
     bool continueRxingTicks = false;

src/core/thread/mesh_forwarder.hpp

@@ -453,6 +453,7 @@ private:
                                  Message::Priority       aPriority);
     Error HandleDatagram(Message &aMessage, const Mac::Address &aMacSource);
     void  ClearReassemblyList(void);
+    Error RemoveUnsecureReassemblyMessage(EvictReason aEvictReason);
     void  HandleDiscoverComplete(void);
 
     void          HandleReceivedFrame(Mac::RxFrame &aFrame);

src/core/thread/mesh_forwarder_ftd.cpp

@@ -180,6 +180,9 @@ Error MeshForwarder::EvictMessage(Message::Priority aPriority, EvictReason aEvic
     Error    error = kErrorNotFound;
     Message *evict = nullptr;
 
+    error = RemoveUnsecureReassemblyMessage(aEvictReason);
+    VerifyOrExit(error == kErrorNotFound);
+
 #if OPENTHREAD_CONFIG_DELAY_AWARE_QUEUE_MANAGEMENT_ENABLE
     error = RemoveAgedMessages();
     VerifyOrExit(error == kErrorNotFound);

src/core/thread/mesh_forwarder_mtd.cpp

@@ -56,11 +56,12 @@ void MeshForwarder::SendMessage(OwnedPtr<Message> aMessagePtr)
 
 Error MeshForwarder::EvictMessage(Message::Priority aPriority, EvictReason aEvictReason)
 {
-    OT_UNUSED_VARIABLE(aEvictReason);
-
     Error    error = kErrorNotFound;
     Message *message;
 
+    error = RemoveUnsecureReassemblyMessage(aEvictReason);
+    VerifyOrExit(error == kErrorNotFound);
+
 #if OPENTHREAD_CONFIG_DELAY_AWARE_QUEUE_MANAGEMENT_ENABLE
     error = RemoveAgedMessages();
     VerifyOrExit(error == kErrorNotFound);