[message] change `GetDirectTransmission()` to `IsDirectTransmission()` (#7510)

diff --git a/src/core/common/message.hpp b/src/core/common/message.hpp
index 37bcaaf..1448cdf 100644
--- a/src/core/common/message.hpp
+++ b/src/core/common/message.hpp
@@ -980,7 +980,7 @@
      * @retval FALSE  If message forwarding is not scheduled for direct transmission.
      *
      */
-    bool GetDirectTransmission(void) const { return GetMetadata().mDirectTx; }
+    bool IsDirectTransmission(void) const { return GetMetadata().mDirectTx; }
 
     /**
      * This method unschedules forwarding using direct transmission.
diff --git a/src/core/thread/mesh_forwarder.cpp b/src/core/thread/mesh_forwarder.cpp
index 80889fe..e527fbf 100644
--- a/src/core/thread/mesh_forwarder.cpp
+++ b/src/core/thread/mesh_forwarder.cpp
@@ -267,7 +267,7 @@
 
     for (curMessage = mSendQueue.GetHead(); curMessage; curMessage = nextMessage)
     {
-        if (!curMessage->GetDirectTransmission())
+        if (!curMessage->IsDirectTransmission())
         {
             nextMessage = curMessage->GetNext();
             continue;
@@ -1017,7 +1017,7 @@
 
     VerifyOrExit(mSendMessage != nullptr);
 
-    OT_ASSERT(mSendMessage->GetDirectTransmission());
+    OT_ASSERT(mSendMessage->IsDirectTransmission());
 
     if (aFrameTxError != kErrorNone)
     {
@@ -1120,7 +1120,7 @@
 
 void MeshForwarder::RemoveMessageIfNoPendingTx(Message &aMessage)
 {
-    VerifyOrExit(!aMessage.GetDirectTransmission() && !aMessage.IsChildPending());
+    VerifyOrExit(!aMessage.IsDirectTransmission() && !aMessage.IsChildPending());
 
     if (mSendMessage == &aMessage)
     {
diff --git a/src/core/thread/mesh_forwarder_ftd.cpp b/src/core/thread/mesh_forwarder_ftd.cpp
index 76775c5..2468f5c 100644
--- a/src/core/thread/mesh_forwarder_ftd.cpp
+++ b/src/core/thread/mesh_forwarder_ftd.cpp
@@ -103,7 +103,7 @@
             }
         }
         else if ((neighbor = Get<NeighborTable>().FindNeighbor(ip6Header.GetDestination())) != nullptr &&
-                 !neighbor->IsRxOnWhenIdle() && !aMessage.GetDirectTransmission())
+                 !neighbor->IsRxOnWhenIdle() && !aMessage.IsDirectTransmission())
         {
             // destined for a sleepy child
             Child &child = *static_cast<Child *>(neighbor);