fix(webdav): Normalize WebDAV error

[liuzs0666]

Summary / 摘要

Normalize error from gowebdav to OpenList's internal error. In this PR only handles ObjectNotFound so internal/op.MakeDir can continue past the existence check and call the actual mkdir API.

Fixes #2564

No user visible change.

• 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 Issues / 关联 Issue

Testing / 测试

• go test ./...
• Manual test / 手动测试:

• Run locally and add WebDAV storage.
• Without this PR, when add new folder, errors out with 404
• With this PR, can add new folder successfully.

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• I have read CONTRIBUTING.
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• 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.
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• This PR includes AI-assisted content.
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