* fix(channels): key WeCom group conversations by chatid, not the sender
Group messages used the sender's userid as chat_id and topic_id, the same
key as the sender's private chat. Wherever no group connection kept them
apart (auth-disabled mode, require_bound_identity: false, legacy group
bindings), a member's private chat and every group they wrote in ran in one
DeerFlow thread. Groups are now keyed by chatid with one thread per member.
A group-scoped connection carries its sender-keyed thread over once; a
legacy binding, which spans every group of the bot, does not.
* docs(changelog): reference #7794 for the WeCom group conversation fix
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>