Set query_type to 'simple' in FoundryAgentTemplate

Replaces the variable query_type with the string 'simple' when constructing the index query payload. This change ensures that the query type is always set to 'simple' for this operation.

Author: Fr4nc3

src/backend/v4/magentic_agents/foundry_agent.py

@@ -178,7 +178,7 @@ async def _create_azure_search_enabled_client(self):
                             {
                                 "index_connection_id": resolved_connection_id,
                                 "index_name": index_name,
-                                "query_type": query_type,
+                                "query_type": "simple",
                             }
                         ]
                     }