[mesh-forwarder] rename method to PrepareNextDirectTransmission() (#7508)

This commit renames the method `MeshForwarder::GetDirectTransmission`
to `PrepareNextDirectTransmission()`. This is to help make this method
different from `Message::GetDirectTransmission()` (which indicate if a
`Message` is marked for direct tx).

src/core/thread/mesh_forwarder.cpp

@@ -246,7 +246,7 @@ void MeshForwarder::ScheduleTransmissionTask(void)
 {
     VerifyOrExit(!mSendBusy && !mTxPaused);
 
-    mSendMessage = GetDirectTransmission();
+    mSendMessage = PrepareNextDirectTransmission();
     VerifyOrExit(mSendMessage != nullptr);
 
     if (mSendMessage->GetOffset() == 0)
@@ -260,7 +260,7 @@ exit:
     return;
 }
 
-Message *MeshForwarder::GetDirectTransmission(void)
+Message *MeshForwarder::PrepareNextDirectTransmission(void)
 {
     Message *curMessage, *nextMessage;
     Error    error = kErrorNone;

src/core/thread/mesh_forwarder.hpp

@@ -429,7 +429,7 @@ private:
                           Ip6::Header &       aIp6Header);
     void     GetMacDestinationAddress(const Ip6::Address &aIp6Addr, Mac::Address &aMacAddr);
     void     GetMacSourceAddress(const Ip6::Address &aIp6Addr, Mac::Address &aMacAddr);
-    Message *GetDirectTransmission(void);
+    Message *PrepareNextDirectTransmission(void);
     void     HandleMesh(uint8_t *             aFrame,
                         uint16_t              aFrameLength,
                         const Mac::Address &  aMacSource,