refactor: use GraphQL string scalar for KSUID transit

Marshal and unmarshal both use the base62 KSUID string, replacing the
legacy JSON object wrapper and aligning gqlgen scalar behavior.

Co-authored-by: Cursor <cursoragent@cursor.com>

ksuid_test.go

@@ -3,6 +3,7 @@ package dbx
 import (
 	"bytes"
 	"encoding/json"
+	"io"
 	"testing"
 
 	"github.com/segmentio/ksuid"
@@ -50,16 +51,32 @@ func TestStringKSUID_Conversion(t *testing.T) {
 
 func TestStringKSUID_GQL(t *testing.T) {
 	id := NewStringKSUID()
+	wire := assertKSUIDGQLMarshal(t, id)
+
+	var parsed StringKSUID
+	require.NoError(t, parsed.UnmarshalGQL(wire))
+	assert.Equal(t, id, parsed)
+}
+
+func TestBinaryKSUID_GQL(t *testing.T) {
+	id := NewBinaryKSUID()
+	wire := assertKSUIDGQLMarshal(t, id)
+
+	var parsed BinaryKSUID
+	require.NoError(t, parsed.UnmarshalGQL(wire))
+	assert.Equal(t, id, parsed)
+}
+
+func assertKSUIDGQLMarshal(t *testing.T, id interface{ String() string; MarshalGQL(w io.Writer) }) string {
+	t.Helper()
+
 	var buf bytes.Buffer
 	id.MarshalGQL(&buf)
 
-	var transport ksuidGQLTransport
-	require.NoError(t, json.Unmarshal(buf.Bytes(), &transport))
-	assert.Equal(t, id.String(), transport.KSUIDStr)
-
-	var parsed StringKSUID
-	require.NoError(t, parsed.UnmarshalGQL(id.String()))
-	assert.Equal(t, id, parsed)
+	var wire string
+	require.NoError(t, json.Unmarshal(buf.Bytes(), &wire))
+	assert.Equal(t, id.String(), wire)
+	return wire
 }
 
 func TestBinaryKSUID_ScanValue(t *testing.T) {
@@ -100,19 +117,6 @@ func TestBinaryKSUID_Conversion(t *testing.T) {
 	assert.Equal(t, b, s.AsBinaryKSUID())
 }
 
-func TestBinaryKSUID_GQL(t *testing.T) {
-	id := NewBinaryKSUID()
-	var buf bytes.Buffer
-	id.MarshalGQL(&buf)
-
-	var transport ksuidGQLTransport
-	require.NoError(t, json.Unmarshal(buf.Bytes(), &transport))
-	assert.Equal(t, id.String(), transport.KSUIDStr)
-
-	var parsed BinaryKSUID
-	require.NoError(t, parsed.UnmarshalGQL(id.String()))
-	assert.Equal(t, id, parsed)
-}
 
 func TestParseStringKSUID(t *testing.T) {
 	id := ksuid.New()