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_ca_exchange_item.go

@@ -9,7 +9,9 @@ import (
 )
 
 // MsgCaExchangeItem represents the MSG_CA_EXCHANGE_ITEM
-type MsgCaExchangeItem struct{}
+type MsgCaExchangeItem struct {
+	AckHandle uint32 // TODO: complete reverse-engineering of request fields
+}
 
 // Opcode returns the ID associated with this packet type.
 func (m *MsgCaExchangeItem) Opcode() network.PacketID {
@@ -18,7 +20,8 @@ func (m *MsgCaExchangeItem) Opcode() network.PacketID {
 
 // Parse parses the packet from binary
 func (m *MsgCaExchangeItem) Parse(bf *byteframe.ByteFrame, ctx *clientctx.ClientContext) error {
-	return errors.New("NOT IMPLEMENTED")
+	m.AckHandle = bf.ReadUint32()
+	return nil
 }
 
 // Build builds a binary packet from the current data.