feat: enhance NebulaGraph pool management & improve Searcher usage logging

[CaralHsi]

Description

Summary:

NebulaGraphDB

• Added shared pool cache & refcounting with thread-safe lock to reuse connections across instances.

• Implemented _make_pool_key, _get_or_create_shared_pool, _release_shared_pool, close_all_shared_pools.

• Updated init to use shared pool acquisition strategy instead of creating new pool each time.

• Adjusted close() to only release shared pool when refcount reaches zero; added safe del.

• Increased default query timeout from 5s → 10s.

• Simplified node property parsing with _build_return_fields helper.

• Added include_embedding option to get_neighbors_by_tag, improved return parsing.

• Refactored multiple query result parsers to unify props extraction.

Searcher

• Introduced dedicated ThreadPoolExecutor(max_workers=4) for async usage updates.

• Changed _update_usage_history to work with batched payloads and safe info snapshotting.

• Added _update_usage_history_worker for async DB updates with exception handling.

• Fixed duplicated rerank top_k scaling in _retrieve_from_long_term_and_user and _retrieve_from_memcubes.

Fix: #(issue)

Docs Issue/PR: (docs-issue-or-pr-link)

Reviewer: @(reviewer)

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人