fix(storage): 修复快速重启时向量存储锁竞争问题

[qin-ctx]

Description

修复快速重启场景下，队列 worker 仍在 drain 消息并尝试写入向量存储时，与正在关闭的后端产生 LOCK 文件竞争的问题。

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• VikingDBManager 提取 mark_closing() 方法，支持在 close() 之前提前标记关闭状态
• OpenVikingService.shutdown() 在停止队列之前先调用 mark_closing()，避免队列 worker 写入已关闭的向量存储
• TextEmbeddingHandler 在 dequeue 入口和 upsert 异常捕获处检查 is_closing 标志，shutdown 期间跳过写入并标记为成功
• 新增 tests/storage/test_collection_schemas.py，覆盖 shutdown 期间跳过 embedding 和 upsert 锁冲突两种场景

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes

在 shutdown 流程中，mark_closing() 被提前调用（在停止队列之前），这样队列 worker 在 drain 剩余消息时能感知到关闭状态，主动跳过向量写入操作，避免与 VikingDBManager.close() 产生锁竞争。

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[MaojiaSheng]

这里为什么是通过 getattr 访问属性

[qin-ctx]

多此一举，待会去掉