Top-level channel messages arrive at _resolve_thread_ts with
metadata.thread_id set to the message's own ts, because the inbound
handler in _handle_message_event uses 'event.ts' as a session-keying
fallback when event.thread_ts is absent. That made metadata alone
insufficient to distinguish a real thread reply from a top-level
message, so reply_in_thread=false only took effect in DMs.
Use reply_to (== incoming message_id == ts for top-level messages) as
the tiebreaker: when metadata.thread_id == reply_to the 'thread' is the
synthetic session-keying fallback, not a real parent, so we reply
directly in the channel. Real thread replies (reply_to != thread_id)
still resolve to the parent thread and preserve conversation context.
Closes#9268.
4b5a88d714