fix(antigravity): 修复批量刷新令牌不设置隐私模式的问题

- refreshSingleAccount ProjectIDMissing 提前返回前补上 EnsureAntigravityPrivacy 调用
- EnsureAntigravityPrivacy 跳过条件从"有任何值"改为"仅 privacy_set 成功时跳过"，
  privacy_set_failed 允许重试，对齐 OpenAI shouldSkipOpenAIPrivacyEnsure 的行为
- 后台 TokenRefreshService.ensureAntigravityPrivacy 同步修改
- ExchangeCode/ValidateRefreshToken 获得令牌后立即调用 setAntigravityPrivacy，
  不依赖后续账号创建流程

Made-with: Cursor

backend/internal/handler/admin/account_handler.go

@@ -839,6 +839,7 @@ func (h *AccountHandler) refreshSingleAccount(ctx context.Context, account *serv
 			if updateErr != nil {
 				return nil, "", fmt.Errorf("failed to update credentials: %w", updateErr)
 			}
+			h.adminService.EnsureAntigravityPrivacy(ctx, updatedAccount)
 			return updatedAccount, "missing_project_id_temporary", nil
 		}