fix(gmp): fix keeper initialization and packet unmarshaling (#8660)

Fix two critical bugs in ICS27-GMP module:

  1. keeper: Initialize msgRouter and accountKeeper fields that were
     being passed to NewKeeper but not assigned to the keeper struct,
     causing nil pointer panics when routing messages

  2. packet: Fix UnmarshalPacketData to use correct Go unmarshal pattern
     by initializing pointer before passing to json.Unmarshal instead of
     passing pointer-to-pointer

Author: mariuszzak

modules/apps/27-gmp/types/packet.go

@@ -77,10 +77,10 @@ func UnmarshalPacketData(bz []byte, ics27Version string, encoding string) (*GMPP
 		panic("unsupported ics27 version")
 	}
 
-	var data *GMPPacketData
+	data := &GMPPacketData{}
 	switch encoding {
 	case EncodingJSON:
-		if err := json.Unmarshal(bz, &data); err != nil {
+		if err := json.Unmarshal(bz, data); err != nil {
 			return nil, errorsmod.Wrapf(ibcerrors.ErrInvalidType, "failed to unmarshal json packet data: %s", err)
 		}
 	case EncodingProtobuf: