fix: prevent duplicate suno task refunds via cas status update

[feitianbubu]

📝 变更描述 / Description

Suno 轮询里失败退款原来是直接调 RefundTaskQuota，没有并发保护。重叠轮询、超时 sweep、多实例同时命中有机率触发重复退款。

改法：同步视频任务的 CAS保护

🚀 变更类型 / Type of change

• 🐛 Bug 修复 (Bug fix) - 请关联对应 Issue，避免将设计取舍、理解偏差或预期不一致直接归类为 bug
• ✨ 新功能 (New feature) - 重大特性建议先通过 Issue 沟通
• ⚡ 性能优化 / 重构 (Refactor)
• 📝 文档更新 (Documentation)

🔗 关联任务 / Related Issue

• 无

✅ 提交前检查项 / Checklist

• 人工确认: 我已亲自整理并撰写此描述，没有直接粘贴未经处理的 AI 输出。
• 非重复提交: 我已搜索现有的 Issues 与 PRs，确认不是重复提交。
• Bug fix 说明: 若此 PR 标记为 Bug fix，我已提交或关联对应 Issue，且不会将设计取舍、预期不一致或理解偏差直接归类为 bug。
• 变更理解: 我已理解这些更改的工作原理及可能影响。
• 范围聚焦: 本 PR 未包含任何与当前任务无关的代码改动。
• 本地验证: 已在本地运行并通过测试或手动验证，维护者可以据此复核结果。
• 安全合规: 代码中无敏感凭据，且符合项目代码规范。

📸 运行证明 / Proof of Work

触发一次suno 生成失败任务, 只退款一次

Summary by CodeRabbit

• Bug Fixes
  • Improved task status updates to prevent conflicting processes from overwriting results.
  • Ensured failed tasks are completed consistently and quota refunds occur only when appropriate.
  • Added clearer handling and logging when task updates fail or are already handled elsewhere.

[coderabbitai]

Walkthrough

Changes

Suno task update flow

Layer / File(s)	Summary
Result application and CAS-protected refund service/task_polling.go	Suno task results now derive failure state, update task timing and status fields, persist through UpdateWithStatus, and refund quota only after a winning failure transition.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant SunoTaskResult
  participant updateSunoTasks
  participant TaskPersistence
  participant RefundTaskQuota
  SunoTaskResult->>updateSunoTasks: provide status and fail reason
  updateSunoTasks->>TaskPersistence: UpdateWithStatus(prevStatus)
  TaskPersistence-->>updateSunoTasks: CAS result
  updateSunoTasks->>RefundTaskQuota: refund non-zero quota after winning failure transition

Loading

Poem

A rabbit watched the task stream flow,
With CAS guarding status below.
If failure won the race just right,
Quota hopped back out of sight.
“No double refunds!” the bunny sings.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: preventing duplicate Suno task refunds with CAS-based status updates.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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