convert: add AST-level check for empty const GenDecl in unit test

Enhance TestNoEmptyConstGroupWhenAllEnumItemsSkipped to also verify
at the Go AST level that no const GenDecl with empty Specs exists
in the package's corresponding ast.File, covering the behavior that
lazy ConstGroup initialization avoids empty const nodes in the AST.

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

@@ -2,6 +2,7 @@ package convert
 
 import (
 	"bytes"
+	goast "go/ast"
 	"go/token"
 	"go/types"
 	"strings"
@@ -252,4 +253,25 @@ func TestNoEmptyConstGroupWhenAllEnumItemsSkipped(t *testing.T) {
 	if constCount != 1 {
 		t.Errorf("expected exactly 1 const block, got %d.\nOutput:\n%s", constCount, output)
 	}
+
+	// Also verify at the AST level: no const GenDecl with empty Specs
+	// should exist in the package's corresponding ast.File.
+	goFile := pkg.p.ASTFile("temp.go")
+	if goFile == nil {
+		t.Fatal("expected ast.File for temp.go, got nil")
+	}
+	constDeclCount := 0
+	for _, decl := range goFile.Decls {
+		genDecl, ok := decl.(*goast.GenDecl)
+		if !ok || genDecl.Tok != token.CONST {
+			continue
+		}
+		constDeclCount++
+		if len(genDecl.Specs) == 0 {
+			t.Error("found empty const declaration in AST (no Specs)")
+		}
+	}
+	if constDeclCount != 1 {
+		t.Errorf("expected exactly 1 const GenDecl in AST, got %d", constDeclCount)
+	}
 }