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_simple_test.go

@@ -290,8 +290,6 @@ func TestEmptyHandlers_NoDb(t *testing.T) {
 		{"handleMsgSysEnumuser", handleMsgSysEnumuser},
 		{"handleMsgSysInfokyserver", handleMsgSysInfokyserver},
 		{"handleMsgMhfGetCaUniqueID", handleMsgMhfGetCaUniqueID},
-		{"handleMsgMhfGetExtraInfo", handleMsgMhfGetExtraInfo},
-		{"handleMsgMhfGetCogInfo", handleMsgMhfGetCogInfo},
 		{"handleMsgMhfStampcardPrize", handleMsgMhfStampcardPrize},
 		{"handleMsgMhfKickExportForce", handleMsgMhfKickExportForce},
 		{"handleMsgSysSetStatus", handleMsgSysSetStatus},