feat(chat): 外部Agent(Dify/n8n/Coze)运行期真实远程调用

- _resolve_market_agents 补齐 platform/agent_endpoint/agent_api_key 字段
- 新增 _call_external_agent 及 dify/n8n/coze 分支; Dify 走SSE流式,兼容
  chat/agent(message)与chatflow/workflow(text_chunk/workflow_finished)事件,
  失败时surface真实错误; n8n走webhook POST; coze走v3 chat+轮询
- send_message 选中带endpoint的外部Agent时直接远程调用,跳过本地LLM扮演
- Conversation新增ext_agent_conversation_id维持Dify/Coze多轮(迁移016)
- 出网统一trust_env=False+_proxy_for_url直连,规避坏代理502

migrations/016_add_conversation_ext_agent.sql

@@ -0,0 +1,14 @@
+-- 016: 为 conversations 增加 ext_agent_conversation_id 列
+-- 用途：外部 Agent(Dify/Coze) 远程调用时保存其平台侧 conversation_id，
+--       以维持多轮上下文。n8n(无平台会话态) 不使用该列。
+-- 幂等：列已存在则跳过。
+SET @col = (SELECT COUNT(*) FROM information_schema.columns
+            WHERE table_schema = DATABASE()
+              AND table_name = 'conversations'
+              AND column_name = 'ext_agent_conversation_id');
+SET @sql = IF(@col = 0,
+    'ALTER TABLE conversations ADD COLUMN ext_agent_conversation_id VARCHAR(128) NULL COMMENT \'外部Agent(Dify/Coze)会话ID，用于维持多轮上下文\'',
+    'SELECT 1');
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;