fix: lazy import faiss in embedding_storage.py

[irmia2026]

Summary

采用与 #8695 一致的懒加载方式处理 embedding_storage.py 中的 import faiss。

Changes

将 import faiss 从模块顶层移至 __init__() 和方法内部，避免 faiss-cpu 1.14.2 C 库在模块加载时触发 CPU 特性检测死锁。

Location	Before	After
Module top	import faiss	移除；TYPE_CHECKING 守卫类型注解
__init__()	直接使用 faiss.	import faiss 在 try/except 内
_read_index()	直接使用 faiss.	方法内 import faiss
_write_index()	直接使用 faiss.	方法内 import faiss

ASCII 临时文件桥接逻辑（_needs_bridge、_safe_temp_dir、_make_temp_file）保持不变。

Related

• [Bug] faiss_memory_index.py 顶层 import faiss 导致 faiss-cpu 1.14.2 下进程挂死 #8695, fix: defer faiss C library import to prevent process hang on startup #8696
• 保留 fix: Faiss read/write failure on Windows with non-ASCII user paths (reopened) #8323（Windows 非 ASCII 路径 Faiss I/O 修复）

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[irmia2026]

Reopened as follow-up per discussion in #8323. Ready for review. 🙏

[sourcery-ai]

Hey - I've left some high level feedback:

• The lazy import faiss pattern is now duplicated in __init__ and implicitly relied on in the static methods; consider centralizing it in a small helper (e.g., _get_faiss() that handles ImportError) to keep behavior and error messaging consistent.
• If _read_index and _write_index are intended to be usable without instantiating EmbeddingStorage, you may want to add explicit ImportError handling in those methods similar to __init__ so callers see a clear, localized error instead of a generic module import failure.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The lazy `import faiss` pattern is now duplicated in `__init__` and implicitly relied on in the static methods; consider centralizing it in a small helper (e.g., `_get_faiss()` that handles ImportError) to keep behavior and error messaging consistent.
- If `_read_index` and `_write_index` are intended to be usable without instantiating `EmbeddingStorage`, you may want to add explicit ImportError handling in those methods similar to `__init__` so callers see a clear, localized error instead of a generic module import failure.

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