feat: add neo4j share-db example

[CaralHsi]

Description

Summary:

Add Shared Database Multi-Tenant Mode to the Neo4j graph store implementation and provide end-to-end working examples for both:

• Dedicated DB Mode (use_multi_db = True)
• Shared DB Mode (use_multi_db = False with user_name)

Highlights:

• Added user_name field for logical tenant separation when using a single database.
• All CRUD and query operations now enforce user_name filters for strict isolation.
• Automatic B-tree index for user_name to maintain query performance.
• Added practical working examples:
  examples/basic_modules/neo4j_example.py
  example_multi_db — shows per-tenant data in separate databases with automatic creation.
  example_shared_db — demonstrates multiple users sharing a single DB with logical separation.
  use_multi_db tree config: examples/data/config/tree_config_shared_database.json

Fix: #79

Reviewer: @J1awei-Yang

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 added necessary documentation (if applicable) | 我已添加必要的文档（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

[J1awei-Yang]

waiting for adding user_name field in config

[CaralHsi]

waiting for adding user_name field in config

done ^_^

[Gaojianli]

没有太看懂这个怎么使用，感觉一般不会直接使用graph，都是使用memos。使用memos的情况下怎么给每个用户修改graph的config呢？有没有example