不支持自定义指定 trace_id

目前 langchain callback handler 的实现不支持自定义 trace_id，也不会解析 langchain msg 的 metadata，希望可以默认支持一下

Locate the LangChain callback handler and inspect how trace IDs are currently created or propagated. Check how LangChain message metadata is represented and identify the relevant test entry points, if present. Done means callers can provide a custom trace_id and message metadata is parsed by default, with tests covering both behaviors.

Written by the indexing model from the issue text.