fix graph insights ignored when partial neo4j results

[Thierry Damiba (thierrypdamiba)]

Summary

• The fusion summary prompt told the LLM to skip Graph Insights if Neo4j returned "no useful results", but it was skipping even when get_related_papers_by_mesh returned 10 results because get_collaborators_with_topics returned 0
• Updated prompt to explicitly list what to look for in each Neo4j tool result (related papers, collaborators, gene co-occurrence)
• Changed instruction to only skip Graph Insights when ALL Neo4j tools return empty
• Filter out empty tool results before passing to the summarizer so the LLM focuses on actual data

Test plan

• Query "BRCA1 breast cancer" and verify Graph Insights section now includes related papers by MeSH
• Query "p53 apoptosis" still shows collaborator networks
• Query with genuinely no graph results still omits Graph Insights