fix(handlers): fix softlock on forge purchases and N-points

MSG_CA_EXCHANGE_ITEM and MSG_MHF_USE_UD_SHOP_COIN had Parse() returning
"NOT IMPLEMENTED". The dispatch loop in handlePacketGroup treats any
Parse error as a silent drop — no ACK is sent, causing the client to
wait indefinitely (softlock). Reported on 9.3.0-rc1 for forge item
purchases and Hunting Road N-point interactions.

Fix follows the pattern from d27da5e: parse only the AckHandle, return
nil from Parse, and respond with doAckBufFail so the client's error
branch exits cleanly without reading response fields.

network/mhfpacket/msg_batch_parse_test.go

@@ -1594,7 +1594,6 @@ func TestBatchParseNotImplemented(t *testing.T) {
 		&MsgSysDispObject{}, &MsgSysHideObject{},
 		&MsgMhfServerCommand{}, &MsgMhfSetLoginwindow{}, &MsgMhfShutClient{},
 		&MsgMhfUpdateGuildcard{},
-		&MsgCaExchangeItem{},
 	}
 
 	for _, pkt := range packets {