Group gating (_should_process_message) read the raw message_thread_id,
while event routing (_build_message_event) normalized it. A plain
non-forum group reply's message_thread_id is a reply-UI anchor, not a
topic, so an anchor id matching an ignored_threads entry wrongly
dropped the message, and the anchor was treated as a routable topic
under allowed_topics.
Extract _effective_message_thread_id and route both gating and
event-building through it, so gating and session routing agree on one
normalized value: real topic/forum messages keep their thread id, reply
anchors are dropped, and forum General-topic messages normalize to the
General-topic id.
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