Refactor DeerFlow to use Gateway's LangGraph-compatible API

- Updated documentation and comments to reflect the transition from LangGraph Server to Gateway.
- Changed default URLs in ChannelManager and tests to point to Gateway.
- Removed references to LangGraph Server in deployment scripts and configurations.
- Updated Nginx configuration to route API traffic to Gateway.
- Adjusted frontend configurations to utilize Gateway's API.
- Removed LangGraph service from Docker Compose files, consolidating services under Gateway.
- Added regression tests to ensure Gateway integration works as expected.

Co-authored-by: Copilot <copilot@github.com>

docker/nginx/nginx.conf

@@ -26,10 +26,6 @@ http {
         server gateway:8001;
     }
 
-    upstream langgraph {
-        server ${LANGGRAPH_UPSTREAM};
-    }
-
     upstream frontend {
         server frontend:3000;
     }
@@ -56,13 +52,11 @@ http {
             return 204;
         }
 
-        # LangGraph API routes
-        # In standard mode: /api/langgraph/* → langgraph:2024 (rewrite to /*)
-        # In gateway mode:  /api/langgraph/* → gateway:8001  (rewrite to /api/*)
-        # Controlled by LANGGRAPH_UPSTREAM and LANGGRAPH_REWRITE env vars.
+        # LangGraph-compatible API routes served by Gateway.
+        # Rewrites /api/langgraph/* to /api/* before proxying to Gateway.
         location /api/langgraph/ {
-            rewrite ^/api/langgraph/(.*) ${LANGGRAPH_REWRITE}$1 break;
-            proxy_pass http://langgraph;
+            rewrite ^/api/langgraph/(.*) /api/$1 break;
+            proxy_pass http://gateway;
             proxy_http_version 1.1;
 
             # Headers