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新增 48 个测试用例覆盖修复代码的各分支路径：
- subscription_maintenance_queue: nil receiver/task、Stop 幂等、零值参数 (+6)
- billing_service: CalculateCostWithConfig、错误传播、SoraImageCost 等 (+12)
- timing_wheel_service: Schedule/ScheduleRecurring after Stop (+3)
- sora_media_cleanup_service: nil guard、Start/Stop 各分支、timezone (+10)
- sora_gateway_service: normalizeSoraMediaURLs、buildSoraContent 等辅助函数 (+17)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

- NewTimingWheelService 改为返回 error，避免 panic

- ProvideTimingWheelService 透传 error 并更新 wire 生成代码

- 补充定时任务调度/取消/周期任务相关单元测试