fix(core): stop node upsert from nulling unset FlowsintType fields

[dextmorgn]

Why

GraphSerializer.graph_node_to_neo4j_dict dumped nodeProperties in full, defaults included. The repository upsert (_build_node_query) does SET n += $props, so every enricher that writes a partial FlowsintType (e.g. Website(url=x)) sent its unset fields as None/falsy defaults and nulled out richer values a prior enricher had already written on the same node. Website and Domain writers hit this routinely: to_webtrackers, to_technologies, to_links, and to_indexed_pages all construct thin Website instances against a node to_website/to_crawler may have already enriched.

nodeProperties now dumps with exclude_unset, so a partial write only sends the fields the caller actually set. The rest of the GraphNode dump (id, x/y, nodeColor, etc.) is untouched, since those fields rely on class defaults being present in every write.

Scope

• flowsint-core/src/flowsint_core/core/graph/serializer.py: GraphSerializer.graph_node_to_neo4j_dict
• flowsint-core/tests/core/graph/test_graph_serializer.py: repro + regression coverage

Blast Radius

Touches every enricher's node write path (create_node/flowsint_type_to_neo4j_dict), since they all funnel through this one function. Low risk: exclude_unset only drops fields a constructor call never set, so nothing that was previously sent on purpose changes. Existing serializer/repository tests (416 in flowsint-core) still pass unmodified.

Verification

python3 -m pytest flowsint-core/tests/ — 416 passed. New tests confirmed failing against the pre-fix code (thin Website leaked active/status_code/etc as explicit keys), then passing after the fix.