fix(feishu): remove root_id fallback from thread_id resolution

[KhanCold]

Problem

In the Feishu adapter, root_id was incorrectly used as a fallback for thread_id:

thread_id = getattr(message, "thread_id", None) or getattr(message, "root_id", None) or None

This is wrong because root_id is the first message in a thread, not a thread identifier.

The result: when a user sends a quoted reply in a P2P DM, Hermes would open a spurious thread session (since root_id is present) instead of continuing the normal DM conversation. This breaks reply threading in private chats.

Fix

Remove the root_id fallback from thread_id resolution. root_id remains legitimately used in reply_to_message_id (it's the root message of the thread being replied to), but it must not leak into session routing.

thread_id = getattr(message, "thread_id", None) or None

Regression Test

Added test_p2p_quote_reply_does_not_use_root_id_as_thread_id in tests/gateway/test_feishu.py that verifies:

• A P2P message with root_id="om_root" and thread_id=None produces event.source.thread_id is None
• reply_to_message_id still correctly captures the parent message

Context

This addresses the issue raised in review comment on #22969 by @sicnuyudidi.

[KhanCold]

Closing this PR as the fix is already being addressed by #20562 (`fix(feishu): avoid threading regular replies`) which covers the same root_id/thread_id issue with regression tests. Thanks for the contribution!