Fixes#8278
When autoThread is enabled and a thread already exists (user continues
conversation in thread), replies were sometimes routing to the root
channel instead of the thread. This happened because the reply delivery
plan only explicitly set the thread target when a NEW thread was created
(createdThreadId), but not when the message was in an existing thread.
The fix adds a fallback case: when threadChannel is set (we're in an
existing thread) but no new thread was created, explicitly route to
the thread's channel ID. This ensures all thread replies go to the
correct destination.