GH-38477: [Go] Fixing decimal 128 rounding issue (#38478)

### Rationale for this change
Fixing an off-by-one rounding issue with decimal128 by ensuring proper precision handling.

### Are these changes tested?
The test case which reproduced the rounding issue has been added as a unit test.

* Closes: #38477

Authored-by: Matt Topol <zotthewizard@gmail.com>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>

go/arrow/decimal128/decimal128.go

@@ -261,7 +261,7 @@ func FromString(v string, prec, scale int32) (n Num, err error) {
 	var precInBits = uint(math.Round(float64(prec+scale+1)/math.Log10(2))) + 1
 
 	var out *big.Float
-	out, _, err = big.ParseFloat(v, 10, 127, big.ToNearestEven)
+	out, _, err = big.ParseFloat(v, 10, 128, big.ToNearestEven)
 	if err != nil {
 		return
 	}
@@ -280,7 +280,7 @@ func FromString(v string, prec, scale int32) (n Num, err error) {
 		// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
 		// we're positive we'll add 0.5.
 		p := (&big.Float{}).SetInt(scaleMultipliers[scale].BigInt())
-		out.Mul(out, p).SetPrec(precInBits)
+		out.SetPrec(precInBits).Mul(out, p)
 		if out.Signbit() {
 			out.Sub(out, pt5)
 		} else {

go/arrow/decimal128/decimal128_test.go

@@ -24,6 +24,7 @@ import (
 
 	"github.com/apache/arrow/go/v15/arrow/decimal128"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestFromU64(t *testing.T) {
@@ -698,3 +699,11 @@ func TestBitLen(t *testing.T) {
 	_, err = decimal128.FromString(b.String(), decimal128.MaxPrecision, -1)
 	assert.ErrorContains(t, err, "bitlen too large for decimal128")
 }
+
+func TestFromStringDecimal128b(t *testing.T) {
+	const decStr = "9323406071781562130.6457232358109488923"
+
+	num, err := decimal128.FromString(decStr, 38, 19)
+	require.NoError(t, err)
+	assert.Equal(t, decStr, num.ToString(19))
+}