convert: add unit test for lazy ConstGroup when all enum items are skipped

Adds TestNoEmptyConstGroupWhenAllEnumItemsSkipped to verify that
no empty const() block is generated when all enum items are already
registered and thus skipped in createEnumItems.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: luoliwoshang <51194195+luoliwoshang@users.noreply.github.com>

Author: xgopilot

cl/internal/convert/package_bulitin_test.go

@@ -1,8 +1,10 @@
 package convert
 
 import (
+	"bytes"
 	"go/token"
 	"go/types"
+	"strings"
 	"testing"
 
 	"github.com/goplus/gogen"
@@ -197,3 +199,57 @@ func TestProcessSymbol(t *testing.T) {
 		}
 	}
 }
+
+func TestNoEmptyConstGroupWhenAllEnumItemsSkipped(t *testing.T) {
+	pnc := cltest.NC(&llcppg.Config{}, nil, cltest.NewConvSym())
+	pkg := emptyPkg(pnc)
+	tempFile := &ncimpl.HeaderFile{
+		File:     "temp.h",
+		FileType: llcppg.Inter,
+	}
+	pkg.p.SetCurFile(tempFile.ToGoFileName("testpkg"), true)
+
+	items := []*ast.EnumItem{
+		{Name: &ast.Ident{Name: "Red"}, Value: &ast.BasicLit{Kind: ast.IntLit, Value: "0"}},
+		{Name: &ast.Ident{Name: "Green"}, Value: &ast.BasicLit{Kind: ast.IntLit, Value: "1"}},
+	}
+
+	// First enum: registers the items normally
+	err := pkg.NewEnumTypeDecl("Color", &ast.EnumTypeDecl{
+		Object: ast.Object{
+			Loc:  &ast.Location{File: "temp.h"},
+			Name: &ast.Ident{Name: "Color"},
+		},
+		Type: &ast.EnumType{Items: items},
+	}, pnc)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Second enum: all items already registered, so all are skipped.
+	// With lazy ConstGroup, no empty const() should be created.
+	err = pkg.NewEnumTypeDecl("Color2", &ast.EnumTypeDecl{
+		Object: ast.Object{
+			Loc:  &ast.Location{File: "temp.h"},
+			Name: &ast.Ident{Name: "Color2"},
+		},
+		Type: &ast.EnumType{Items: items},
+	}, pnc)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var buf bytes.Buffer
+	err = pkg.p.WriteTo(&buf, "temp.go")
+	if err != nil {
+		t.Fatal(err)
+	}
+	output := buf.String()
+
+	// There should be exactly one const block from the first enum.
+	// No empty const() from the second enum.
+	constCount := strings.Count(output, "const (")
+	if constCount != 1 {
+		t.Errorf("expected exactly 1 const block, got %d.\nOutput:\n%s", constCount, output)
+	}
+}