fix: Neo4j vector search pre-filter and metadata handling

[CaralHsi]

Description

Fix Neo4j vector search returning empty results in shared-database multi-tenant mode, along with several related metadata handling bugs.

Core fix: Replace post-filter approach (db.index.vector.queryNodes + WHERE) with Neo4j 5.18+ pre-filtering (MATCH + WHERE + vector.similarity.cosine()). This ensures filters are applied before similarity computation, so target user's nodes are never excluded by a global top-k truncation.

Additional fixes:

• metadata["sources"] → metadata.get("sources") to prevent KeyError when sources is absent
• _parse_node sources deserialization: [0] == "}" → [-1] == "}" (was checking first char instead of last, making json.loads never execute)
• neo4j_community.py add_node: gracefully skip vector DB insert when embedding is None instead of raising ValueError
• neo4j_community.py add_nodes_batch: skip VecDBItem creation for nodes without embeddings

Related Issue (Required): Fixes #1360

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Unit Test
• Test Script Or Test Steps (please provide)

Unit tests (6 tests, mocked Neo4j driver):

• TestVectorSearchPreFilter: Verify pre-filter path uses MATCH + vector.similarity.cosine() when WHERE clauses present, and ANN path uses queryNodes when no filters
• TestSourcesKeyErrorRegression: Verify add_node and _parse_node work without sources key

Integration tests (3 tests, real Neo4j 5.18+):

• TestNeo4jPreFilterIntegration: Insert 50 nodes for other users + 3 for target user, verify search with user_name filter returns all 3 target user nodes

Run: pytest tests/graph_dbs/test_neo4j_vector_search.py -v

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

Reviewer Checklist

• closes Neo4j vector search returns empty results in shared-database multi-tenant mode #1360
• Made sure Checks passed
• Tests have been provided