test: 补全品牌方平台规则 API 测试覆盖 (22 个新测试)

- TestBrandPlatformRuleParse: 文档解析 6 个用例 (201/400/降级)
- TestBrandPlatformRuleConfirm: 确认生效 5 个用例 (active/编辑/停旧/404/隔离)
- TestBrandPlatformRuleList: 列表查询 5 个用例 (空/筛选/隔离)
- TestBrandPlatformRuleDelete: 删除 4 个用例 (204/实际删除/404/隔离)
- TestBrandPlatformRuleLifecycle: 完整生命周期 1 个用例
- 修复 confirm 端点 flush 后缺少 refresh 导致的 MissingGreenlet 错误

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/api/rules.py

@@ -611,6 +611,7 @@ async def confirm_platform_rule(
     rule.parsed_rules = request.parsed_rules.model_dump()
     rule.status = RuleStatus.ACTIVE.value
     await db.flush()
+    await db.refresh(rule)
 
     return _format_platform_rule(rule)