migrates some helper functions from beacon API to apiutil (#15125)

* migration and changelog

* missed valid root

* removing unneeded test, more optimization

* adding strict dependency

* linting

* fixing 1 more test

* bastin's suggestion

encoding/bytesutil/hex_test.go

@@ -3,6 +3,7 @@ package bytesutil_test
 import (
 	"testing"
 
+	fieldparams "github.com/prysmaticlabs/prysm/v5/config/fieldparams"
 	"github.com/prysmaticlabs/prysm/v5/encoding/bytesutil"
 	"github.com/prysmaticlabs/prysm/v5/testing/assert"
 )
@@ -27,3 +28,53 @@ func TestIsHex(t *testing.T) {
 		assert.Equal(t, tt.b, isHex)
 	}
 }
+
+func TestDecodeHexWithLength_Root(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		valid bool
+	}{
+		{
+			name:  "correct format",
+			input: "0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2",
+			valid: true,
+		},
+		{
+			name:  "root too small",
+			input: "0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f",
+			valid: false,
+		},
+		{
+			name:  "root too big",
+			input: "0xcf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f22",
+			valid: false,
+		},
+		{
+			name:  "empty root",
+			input: "",
+			valid: false,
+		},
+		{
+			name:  "no 0x prefix",
+			input: "cf8e0d4e9587369b2301d0790347320302cc0943d5a1884560367e8208d920f2",
+			valid: false,
+		},
+		{
+			name:  "invalid characters",
+			input: "0xzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz",
+			valid: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, err := bytesutil.DecodeHexWithLength(tt.input, fieldparams.RootLength)
+			if tt.valid {
+				assert.NoError(t, err)
+			} else {
+				assert.Equal(t, true, err != nil)
+			}
+		})
+	}
+}