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>

frontend/src/core/api/api-client.ts

@@ -13,8 +13,8 @@ import { sanitizeRunStreamOptions } from "./stream-mode";
  *
  * Reading the cookie per-request (rather than baking it into the SDK's
  * ``defaultHeaders`` at construction) handles login / logout / password
- * change cookie rotation transparently. Both the ``/langgraph-compat/*``
- * SDK path and the direct REST endpoints in ``fetcher.ts:fetchWithAuth``
+ * change cookie rotation transparently. Both the ``/api/langgraph/*`` SDK
+ * path and the direct REST endpoints in ``fetcher.ts:fetchWithAuth``
  * share :func:`readCsrfCookie` and :const:`STATE_CHANGING_METHODS` so
  * the contract stays in lockstep.
  */
@@ -32,8 +32,10 @@ function injectCsrfHeader(_url: URL, init: RequestInit): RequestInit {
 }
 
 function createCompatibleClient(isMock?: boolean): LangGraphClient {
+  const apiUrl = getLangGraphBaseURL(isMock);
+  console.log(`Creating API client with base URL: ${apiUrl}`);
   const client = new LangGraphClient({
-    apiUrl: getLangGraphBaseURL(isMock),
+    apiUrl,
     onRequest: injectCsrfHeader,
   });