fix(bigquery): parse negative NUMERIC from arrow (googleapis#11052)

The `big.Int.SetBytes` method interprets input as an unsigned number, so we lose sign information when parsing using. So Arrow decimal parsing code is simplified by just relying on `big.Rat.SetString` method for parsing. 

Towards internal b/376149769

bigquery/arrow.go

@@ -241,18 +241,14 @@ func convertArrowValue(col arrow.Array, i int, ft arrow.DataType, fs *FieldSchem
 	case *arrow.Decimal128Type:
 		dft := ft.(*arrow.Decimal128Type)
 		v := col.(*array.Decimal128).Value(i)
-		rat := big.NewRat(1, 1)
-		rat.Num().SetBytes(v.BigInt().Bytes())
-		d := rat.Denom()
-		d.Exp(big.NewInt(10), big.NewInt(int64(dft.Scale)), nil)
+		rat := new(big.Rat)
+		rat.SetString(v.ToString(dft.Scale))
 		return Value(rat), nil
 	case *arrow.Decimal256Type:
 		dft := ft.(*arrow.Decimal256Type)
 		v := col.(*array.Decimal256).Value(i)
-		rat := big.NewRat(1, 1)
-		rat.Num().SetBytes(v.BigInt().Bytes())
-		d := rat.Denom()
-		d.Exp(big.NewInt(10), big.NewInt(int64(dft.Scale)), nil)
+		rat := new(big.Rat)
+		rat.SetString(v.ToString(dft.Scale))
 		return Value(rat), nil
 	case *arrow.ListType:
 		arr := col.(*array.List)

bigquery/integration_test.go

@@ -2167,6 +2167,7 @@ func initQueryParameterTestCases() {
 	dtm := civil.DateTime{Date: d, Time: tm}
 	ts := time.Date(2016, 3, 20, 15, 04, 05, 0, time.UTC)
 	rat := big.NewRat(13, 10)
+	nrat := big.NewRat(-13, 10)
 	bigRat := big.NewRat(12345, 10e10)
 	rangeTimestamp1 := &RangeValue{
 		Start: time.Date(2016, 3, 20, 15, 04, 05, 0, time.UTC),
@@ -2208,6 +2209,13 @@ func initQueryParameterTestCases() {
 			[]Value{rat},
 			rat,
 		},
+		{
+			"NegativeBigRatParam",
+			"SELECT @val",
+			[]QueryParameter{{Name: "val", Value: nrat}},
+			[]Value{nrat},
+			nrat,
+		},
 		{
 			"BoolParam",
 			"SELECT @val",