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>

backend/tests/test_channels.py

@@ -495,7 +495,7 @@ class TestChannelManager:
             await _wait_for(lambda: len(outbound_received) >= 1)
             await manager.stop()
 
-            # Thread should be created on the LangGraph Server
+            # Thread should be created through Gateway
             mock_client.threads.create.assert_called_once()
 
             # Thread ID should be stored
@@ -1987,28 +1987,28 @@ class TestChannelService:
     def test_service_urls_fall_back_to_env(self, monkeypatch):
         from app.channels.service import ChannelService
 
-        monkeypatch.setenv("DEER_FLOW_CHANNELS_LANGGRAPH_URL", "http://langgraph:2024")
+        monkeypatch.setenv("DEER_FLOW_CHANNELS_LANGGRAPH_URL", "http://gateway:8001/api")
         monkeypatch.setenv("DEER_FLOW_CHANNELS_GATEWAY_URL", "http://gateway:8001")
 
         service = ChannelService(channels_config={})
 
-        assert service.manager._langgraph_url == "http://langgraph:2024"
+        assert service.manager._langgraph_url == "http://gateway:8001/api"
         assert service.manager._gateway_url == "http://gateway:8001"
 
     def test_config_service_urls_override_env(self, monkeypatch):
         from app.channels.service import ChannelService
 
-        monkeypatch.setenv("DEER_FLOW_CHANNELS_LANGGRAPH_URL", "http://langgraph:2024")
+        monkeypatch.setenv("DEER_FLOW_CHANNELS_LANGGRAPH_URL", "http://gateway:8001/api")
         monkeypatch.setenv("DEER_FLOW_CHANNELS_GATEWAY_URL", "http://gateway:8001")
 
         service = ChannelService(
             channels_config={
-                "langgraph_url": "http://custom-langgraph:2024",
+                "langgraph_url": "http://custom-gateway:8001/api",
                 "gateway_url": "http://custom-gateway:8001",
             }
         )
 
-        assert service.manager._langgraph_url == "http://custom-langgraph:2024"
+        assert service.manager._langgraph_url == "http://custom-gateway:8001/api"
         assert service.manager._gateway_url == "http://custom-gateway:8001"
 
     def test_disabled_channel_with_string_creds_emits_warning(self, caplog):