[mesh-forwarder] use RemoveMessageIfNoPendingTx() on direct tx fails (#10471)

This commit updates `PrepareNextDirectTransmission()` to use
`RemoveMessageIfNoPendingTx()` if a message cannot be prepared for
direct transmission. This ensures the message is not dequeued and
freed if it is also queued for indirect transmission.

src/core/thread/mesh_forwarder.cpp

@@ -654,7 +654,7 @@ Message *MeshForwarder::PrepareNextDirectTransmission(void)
 #endif
             LogMessage(kMessageDrop, *curMessage, error);
             FinalizeMessageDirectTx(*curMessage, error);
-            mSendQueue.DequeueAndFree(*curMessage);
+            RemoveMessageIfNoPendingTx(*curMessage);
             continue;
         }
     }