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>

config.example.yaml

@@ -868,18 +868,18 @@ run_events:
 # All channels use outbound connections (WebSocket or polling) — no public IP required.
 
 # channels:
-#   # LangGraph Server URL for thread/message management (default: http://localhost:2024)
+#   # LangGraph-compatible Gateway API base URL for thread/message management (default: http://localhost:8001/api)
 #   # For Docker deployments, use the Docker service name instead of localhost:
-#   #   langgraph_url: http://langgraph:2024
+#   #   langgraph_url: http://gateway:8001/api
 #   #   gateway_url: http://gateway:8001
-#   langgraph_url: http://localhost:2024
+#   langgraph_url: http://localhost:8001/api
 #   # Gateway API URL for auxiliary queries like /models, /memory (default: http://localhost:8001)
 #   gateway_url: http://localhost:8001
 #   #
 #   # Docker Compose note:
 #   # If channels run inside the gateway container, use container DNS names instead
 #   # of localhost, for example:
-#   # langgraph_url: http://langgraph:2024
+#   # langgraph_url: http://gateway:8001/api
 #   # gateway_url: http://gateway:8001
 #   # You can also set DEER_FLOW_CHANNELS_LANGGRAPH_URL / DEER_FLOW_CHANNELS_GATEWAY_URL.
 #