fix: add feePayer as signer

x/tx/decode/decode.go

@@ -167,6 +167,18 @@ func (d *Decoder) Decode(txBytes []byte) (*DecodedTx, error) {
 		}
 	}
 
+	// If a fee payer is specified in the AuthInfo, it must be added to the list of signers
+	if authInfo.Fee.Payer != "" {
+		feeAddr, err := d.signingCtx.AddressCodec().StringToBytes(authInfo.Fee.Payer)
+		if err != nil {
+			return nil, errorsmod.Wrap(ErrTxDecode, err.Error())
+		}
+
+		if _, seen := seenSigners[string(feeAddr)]; !seen {
+			signers = append(signers, feeAddr)
+		}
+	}
+
 	return &DecodedTx{
 		Messages:                     msgs,
 		DynamicMessages:              dynamicMsgs,