fix: return appropriate result type for chunks search in mcp (#777)

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## Description
Add CHUNKS search handling and better unknown search type return value

## DCO Affirmation
I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin.

cognee-mcp/src/server.py

@@ -196,9 +196,13 @@ async def search(search_query: str, search_type: str) -> str:
             return json.dumps(search_results, cls=JSONEncoder)
         elif search_type.upper() == "GRAPH_COMPLETION" or search_type.upper() == "RAG_COMPLETION":
             return search_results[0]
-        else:
+        elif search_type.upper() == "CHUNKS":
+            return str(search_results)
+        elif search_type.upper() == "INSIGHTS":
             results = retrieved_edges_to_string(search_results)
             return results
+        else:
+            return str(search_results)
 
 
 async def prune():