fix(mcp): persist MCP sessions across tool calls for stateful servers

MCP tools loaded via langchain-mcp-adapters created a new session on
  every call, causing stateful servers like Playwright to lose browser
  state (pages, forms) between consecutive tool invocations within the
  same thread.

  Add MCPSessionPool that maintains persistent sessions scoped by
  (server_name, thread_id). Tool calls within the same thread now reuse
  the same MCP session, preserving server-side state. Sessions are evicted
  in LRU order (max 256) and cleaned up on cache invalidation.

  Fixes #3054

backend/packages/harness/deerflow/mcp/cache.py

@@ -134,9 +134,36 @@ def reset_mcp_tools_cache() -> None:
     """Reset the MCP tools cache.
 
     This is useful for testing or when you want to reload MCP tools.
+    Also closes all persistent MCP sessions so they are recreated on
+    the next tool load.
     """
     global _mcp_tools_cache, _cache_initialized, _config_mtime
     _mcp_tools_cache = None
     _cache_initialized = False
     _config_mtime = None
+
+    # Close persistent sessions – they will be recreated by the next
+    # get_mcp_tools() call with the (possibly updated) connection config.
+    try:
+        from deerflow.mcp.session_pool import get_session_pool
+
+        pool = get_session_pool()
+        try:
+            loop = asyncio.get_event_loop()
+            if loop.is_running():
+                import concurrent.futures
+
+                with concurrent.futures.ThreadPoolExecutor() as executor:
+                    future = executor.submit(asyncio.run, pool.close_all())
+                    future.result()
+            else:
+                loop.run_until_complete(pool.close_all())
+        except RuntimeError:
+            asyncio.run(pool.close_all())
+    except Exception:
+        logger.debug("Could not close MCP session pool on cache reset", exc_info=True)
+
+    from deerflow.mcp.session_pool import reset_session_pool
+
+    reset_session_pool()
     logger.info("MCP tools cache reset")