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.

server/channelserver/handlers_simple_test.go

@@ -279,7 +279,6 @@ func TestEmptyHandlers_NoDb(t *testing.T) {
 		{"handleMsgSysUpdateRight", handleMsgSysUpdateRight},
 		{"handleMsgSysAuthQuery", handleMsgSysAuthQuery},
 		{"handleMsgSysAuthTerminal", handleMsgSysAuthTerminal},
-		{"handleMsgCaExchangeItem", handleMsgCaExchangeItem},
 		{"handleMsgMhfServerCommand", handleMsgMhfServerCommand},
 		{"handleMsgMhfSetLoginwindow", handleMsgMhfSetLoginwindow},
 		{"handleMsgSysTransBinary", handleMsgSysTransBinary},
@@ -294,7 +293,6 @@ func TestEmptyHandlers_NoDb(t *testing.T) {
 		{"handleMsgMhfKickExportForce", handleMsgMhfKickExportForce},
 		{"handleMsgSysSetStatus", handleMsgSysSetStatus},
 		{"handleMsgSysEcho", handleMsgSysEcho},
-		{"handleMsgMhfUseUdShopCoin", handleMsgMhfUseUdShopCoin},
 		{"handleMsgMhfEnterTournamentQuest", handleMsgMhfEnterTournamentQuest},
 	}