GH-35965: [Go] Fix Decimal256DictionaryBuilder (#35966)

### Rationale for this change
`Decimal256DictionaryBuilder` cannot append `decimal256` value and prefers `decimal128` instead.
```
cannot use v (variable of type decimal256.Num) as decimal128.Num value in argument to builder.Append
```

### What changes are included in this PR?

### Are these changes tested?
Yes

### Are there any user-facing changes?
Yes

* Closes: #35965

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

go/arrow/array/dictionary.go

@@ -28,6 +28,7 @@ import (
 	"github.com/apache/arrow/go/v13/arrow"
 	"github.com/apache/arrow/go/v13/arrow/bitutil"
 	"github.com/apache/arrow/go/v13/arrow/decimal128"
+	"github.com/apache/arrow/go/v13/arrow/decimal256"
 	"github.com/apache/arrow/go/v13/arrow/float16"
 	"github.com/apache/arrow/go/v13/arrow/internal/debug"
 	"github.com/apache/arrow/go/v13/arrow/memory"
@@ -45,12 +46,12 @@ import (
 //
 // For example, the array:
 //
-//      ["foo", "bar", "foo", "bar", "foo", "bar"]
+//	["foo", "bar", "foo", "bar", "foo", "bar"]
 //
 // with dictionary ["bar", "foo"], would have the representation of:
 //
-//      indices: [1, 0, 1, 0, 1, 0]
-//      dictionary: ["bar", "foo"]
+//	indices: [1, 0, 1, 0, 1, 0]
+//	dictionary: ["bar", "foo"]
 //
 // The indices in principle may be any integer type.
 type Dictionary struct {
@@ -883,6 +884,11 @@ func getvalFn(arr arrow.Array) func(i int) interface{} {
 			val := typedarr.Value(i)
 			return (*(*[arrow.Decimal128SizeBytes]byte)(unsafe.Pointer(&val)))[:]
 		}
+	case *Decimal256:
+		return func(i int) interface{} {
+			val := typedarr.Value(i)
+			return (*(*[arrow.Decimal256SizeBytes]byte)(unsafe.Pointer(&val)))[:]
+		}
 	case *DayTimeInterval:
 		return func(i int) interface{} {
 			val := typedarr.Value(i)
@@ -1373,7 +1379,7 @@ type Decimal256DictionaryBuilder struct {
 	dictionaryBuilder
 }
 
-func (b *Decimal256DictionaryBuilder) Append(v decimal128.Num) error {
+func (b *Decimal256DictionaryBuilder) Append(v decimal256.Num) error {
 	return b.appendValue((*(*[arrow.Decimal256SizeBytes]byte)(unsafe.Pointer(&v)))[:])
 }
 func (b *Decimal256DictionaryBuilder) InsertDictValues(arr *Decimal256) (err error) {

go/arrow/array/dictionary_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/apache/arrow/go/v13/arrow/array"
 	"github.com/apache/arrow/go/v13/arrow/bitutil"
 	"github.com/apache/arrow/go/v13/arrow/decimal128"
+	"github.com/apache/arrow/go/v13/arrow/decimal256"
 	"github.com/apache/arrow/go/v13/arrow/memory"
 	"github.com/apache/arrow/go/v13/internal/types"
 	"github.com/stretchr/testify/assert"
@@ -878,7 +879,7 @@ func TestFixedSizeBinaryDictionaryStringRoundTrip(t *testing.T) {
 	assert.True(t, array.Equal(arr, arr1))
 }
 
-func TestDecimalDictionaryBuilderBasic(t *testing.T) {
+func TestDecimal128DictionaryBuilderBasic(t *testing.T) {
 	mem := memory.NewCheckedAllocator(memory.DefaultAllocator)
 	defer mem.AssertSize(t, 0)
 
@@ -906,6 +907,34 @@ func TestDecimalDictionaryBuilderBasic(t *testing.T) {
 	assert.True(t, array.ArrayApproxEqual(expected, result))
 }
 
+func TestDecimal256DictionaryBuilderBasic(t *testing.T) {
+	mem := memory.NewCheckedAllocator(memory.DefaultAllocator)
+	defer mem.AssertSize(t, 0)
+
+	test := []decimal256.Num{decimal256.FromI64(12), decimal256.FromI64(12), decimal256.FromI64(11), decimal256.FromI64(12)}
+	dictType := &arrow.DictionaryType{IndexType: &arrow.Int8Type{}, ValueType: &arrow.Decimal256Type{Precision: 2, Scale: 0}}
+	bldr := array.NewDictionaryBuilder(mem, dictType)
+	defer bldr.Release()
+
+	builder := bldr.(*array.Decimal256DictionaryBuilder)
+	for _, v := range test {
+		assert.NoError(t, builder.Append(v))
+	}
+
+	result := bldr.NewDictionaryArray()
+	defer result.Release()
+
+	indices, _, _ := array.FromJSON(mem, dictType.IndexType, strings.NewReader("[0, 0, 1, 0]"))
+	defer indices.Release()
+	dict, _, _ := array.FromJSON(mem, dictType.ValueType, strings.NewReader("[12, 11]"))
+	defer dict.Release()
+
+	expected := array.NewDictionaryArray(dictType, indices, dict)
+	defer expected.Release()
+
+	assert.True(t, array.ArrayApproxEqual(expected, result))
+}
+
 func TestNullDictionaryBuilderBasic(t *testing.T) {
 	mem := memory.NewCheckedAllocator(memory.DefaultAllocator)
 	defer mem.AssertSize(t, 0)