fix(chunk): handle missing chunk part zero

[jyxjjj]

Summary / 摘要

• Return ObjectNotFound when a chunk directory exists but lacks a valid part 0.

• Avoid nil first dereference in Chunk.Get() for invalid chunk directories.

• Ignore invalid negative chunk part indexes while scanning chunk directories.

• No public API, config, storage format, migration, or related repository changes.

• This PR has breaking changes.
  / 此 PR 包含破坏性变更。

• This PR changes public API, config, storage format, or migration behavior.
  / 此 PR 修改了公开 API、配置、存储格式或迁移行为。

• This PR requires corresponding changes in related repositories.
  / 此 PR 需要关联仓库同步修改。

Related repository PRs / 关联仓库 PR:

• OpenList-Frontend: N/A
• OpenList-Docs: N/A

Related Issues / 关联 Issue

Fixes #2618

Testing / 测试

• go test ./...
• go test ./drivers/chunk
• Manual test / 手动测试: Not run; the failing path is covered by code inspection and the package compile test.

Checklist / 检查清单

• I have read CONTRIBUTING.
  / 我已阅读 CONTRIBUTING。
• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
  / 我确认此贡献符合仓库许可证、贡献规范和行为准则。
• I have formatted the changed code with gofmt, go fmt, or prettier where applicable.
  / 我已按适用情况使用 gofmt、go fmt 或 prettier 格式化变更代码。
• I have requested review from relevant maintainers or code owners where applicable.
  / 我已在适用情况下请求相关维护者或代码所有者审查。

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• This PR includes AI-assisted content.
  / 此 PR 包含 AI 辅助内容。

Tools used / 使用工具:

• ChatGPT
• Codex
• GitHub Copilot
• Claude
• Gemini
• Other (please specify) / 其他（请注明）:

Usage scope / 使用范围:

• Code generation / 代码生成

• Refactoring / 重构

• Documentation / 文档

• Tests / 测试

• Translation / 翻译

• Review assistance / 审查辅助

• I have reviewed and validated all AI-assisted content included in this PR.
  / 我已审核并验证此 PR 中的所有 AI 辅助内容。

• I have ensured that all AI-assisted commits include Co-Authored-By attribution.
  / 我已确保所有 AI 辅助提交都包含 Co-Authored-By 归属信息。

• I can reproduce all AI-assisted content included in this PR without any AI tools.
  / 我可以在没有任何 AI 工具的情况下重现此 PR 中包含的所有 AI 辅助内容。

[jyxjjj]

用户已确认，和我想的一致，可以进行review

[jyxjjj]

本修复不能解决文件夹存在的问题，必须手动删除
只修复了会正常返回错误
我暂时不认为OpenList应该处理用户手动删除文件的问题
如有不同意见可继续新的PR。