refactor: remove unused ClientSecret constant

The ClientSecret constant was left as an empty string after
getClientSecret() was refactored to use defaultClientSecret.
Remove the dead constant and update the test accordingly.

backend/internal/pkg/antigravity/oauth_test.go

@@ -680,9 +680,6 @@ func TestConstants_值正确(t *testing.T) {
 	if ClientID != "1071006060591-tmhssin2h21lcre235vtolojh4g403ep.apps.googleusercontent.com" {
 		t.Errorf("ClientID 不匹配: got %s", ClientID)
 	}
-	if ClientSecret != "" {
-		t.Error("ClientSecret 常量应为空字符串（默认值已移至 defaultClientSecret）")
-	}
 	secret, err := getClientSecret()
 	if err != nil {
 		t.Fatalf("getClientSecret 应返回默认值，但报错: %v", err)