KGVisualizer.visualize_network() (and sibling methods) only accepted a raw
dict. Passing a KnowledgeGraph object — the natural output of
GraphBuilder.build() — silently returned without rendering.
Added _normalize_graph() which duck-types the input: dicts pass through
unchanged; any object exposing .entities / .relationships attributes is
converted to the canonical dict form; anything else raises a clear
ProcessingError naming the offending type.
_normalize_graph() is called as the first statement in visualize_network(),
visualize_communities(), visualize_centrality(), visualize_entity_types(),
and visualize_relationship_matrix().
Also adds 21 tests in tests/visualization/test_kg_visualizer_normalize_graph.py
covering the helper directly, the end-to-end regression for #458, and
a guard that every public method routes through _normalize_graph.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
