fix(feishu): resolve thread root message_id for topic reply

[LigHt3r2]

Summary

Fix the Feishu bot reply behavior in topic/thread group chats.
Currently bot replies appear as standalone messages instead of threaded replies.

Problem

When _send_raw_message() receives a message with thread_id metadata
in Feishu topic groups, the existing code treats thread_id (format
omt_xxx) as receive_id for CreateMessage, which creates a new
standalone message instead of a reply in the thread.

Solution

1. Add _get_thread_root_message_id() method: uses Feishu SDK's
   ListMessageRequest API (container_id_type="thread") to list messages
   in the thread, identifies the root message (where root_id is absent
   or equals its own message_id).
2. Modify the thread branch in _send_raw_message(): first resolve the
   root message ID, then call the Reply API with reply_in_thread=true,
   so the response correctly appears as a threaded reply.
3. Returns explicit SendResult error on resolution failure, no silent
   fallback.

Changes

• plugins/platforms/feishu/adapter.py: +50 / -8 lines
  • New: _get_thread_root_message_id() method
  • Modified: _send_raw_message() thread_id branch

Testing

• Bot responds as threaded reply in Feishu topic groups
• No regression in regular group chats and DMs
• Resolution failure returns clear error without affecting other sends