fix(graph): preserve existing node ntype and props in add_node

[Kaap10]

Description

Fixes a critical data-loss bug in the Knowledge Graph layer where calling add_edge() or link_docs() inadvertently erased pre-existing node ntype and props metadata in DynamoDB.

Related issue

Fixes #271

Changes

• src/dynavec/graph.py: Updated GraphStore.add_node() to conditionally set ntype and props using DynamoDB if_not_exists clauses instead of unconditionally replacing them with None / {}.
• tests/test_graph_properties.py: Added unit tests verifying:
  • add_edge() and link_docs() preserve existing entity ntype and props.
  • Partial add_node() calls preserve unspecified node attributes.

Testing

• Tests pass locally (pytest tests/test_graph_properties.py tests/test_graph_delete.py tests/test_graph_export.py — 32 passed)
• Ruff checks pass
• Documentation updated, if applicable

Checklist

• My changes are focused and relevant to this pull request.
• I have added or updated tests where appropriate.
• I have reviewed my changes for unrelated modifications.
• I have updated documentation where necessary.

[codeforstartups]

Reviewed and verified — this is a real data-loss fix, nicely done. 🎯

add_edge() (graph.py:218-219) and link_docs() (graph.py:238) both call add_node(ns, id) with no ntype/props, so the old unconditional SET ntype = :t, props = :p silently reset an entity's type to None and its props to {} every time an edge or doc-link touched it. Switching the unspecified attributes to if_not_exists(...) preserves them, while an explicit value still overwrites — which your partial-update test pins down nicely.

The two new tests (test_add_edge_and_link_docs_preserve_existing_node_metadata, test_add_node_partial_update_preserves_unspecified_attributes) cover exactly the regression. All 4 CI checks green (ruff + strict mypy + pytest on 3.9/3.11/3.12). Merging. Thanks @Kaap10!