chore: set default mcp_source to 'custom' if None

--bug=1061187 --user=刘瑞斌 【迁移】应用AI对话节点使用了MCP，迁移后对话时，不会调用MCP工具 https://www.tapd.cn/62980211/s/1765228

Author: liuruibin

apps/application/flow/step_node/ai_chat_step_node/impl/base_chat_node.py

@@ -271,6 +271,9 @@ def _handle_mcp_request(self, mcp_enable, tool_enable, mcp_source, mcp_servers,
 
         mcp_servers_config = {}
 
+        # 迁移过来mcp_source是None
+        if mcp_source is None:
+            mcp_source = 'custom'
         if mcp_enable:
             if mcp_source == 'custom' and mcp_servers is not None and '"stdio"' not in mcp_servers:
                 mcp_servers_config = json.loads(mcp_servers)