fix(researcher): Handle non-string search results in summarizer node

backend/src/ea_chatbot/graph/workers/researcher/nodes/summarizer.py

@@ -17,7 +17,15 @@ def summarizer_node(state: WorkerState) -> dict:
         callbacks=[LangChainLoggingHandler(logger=logger)]
     )
     
-    results_str = "\n---\n".join(raw_results)
+    # Ensure all results are strings (Gemini/OpenAI might return complex content)
+    processed_results = []
+    for res in raw_results:
+        if isinstance(res, list):
+            processed_results.append(str(res))
+        else:
+            processed_results.append(str(res))
+
+    results_str = "\n---\n".join(processed_results)
     
     prompt = f"""You are a Research Specialist sub-agent. You have completed a research sub-task.
 Task: {task}