fix(items): stop G-rank Workshop/Cog softlock on missing ACK

MSG_MHF_GET_EXTRA_INFO (0xA6) and MSG_MHF_GET_COG_INFO (0xC3) had
Parse() returning NOT IMPLEMENTED. The dispatch loop treats any Parse
error as a hard drop — no ACK is ever sent, so the client waits
indefinitely and effectively soft-locks when entering the G-rank
Workshop or Master Felyne (Cog) screens.

Fix: parse AckHandle (the only field we can confirm from the protocol)
and respond with doAckBufFail so the client receives a well-formed
buf-type ACK with error code 1. The client's fail branch for these
requests exits cleanly without reading response fields, avoiding the
read-past-EOF crash that an empty success ACK would cause.

The full response format for both packets is still unknown; a complete
implementation requires further RE. The TODO comments mark the gap.

Fixes #180.

server/channelserver/handlers_items_test.go

@@ -494,13 +494,8 @@ func TestHandleMsgMhfGetExtraInfo(t *testing.T) {
 	server := createMockServer()
 	session := createMockSession(1, server)
 
-	defer func() {
-		if r := recover(); r != nil {
-			t.Errorf("handleMsgMhfGetExtraInfo panicked: %v", r)
-		}
-	}()
-
-	handleMsgMhfGetExtraInfo(session, nil)
+	pkt := &mhfpacket.MsgMhfGetExtraInfo{AckHandle: 1}
+	handleMsgMhfGetExtraInfo(session, pkt)
 }
 
 func TestHandleMsgMhfTransferItem(t *testing.T) {