Group threads/followUp always steered to members[0], so a follow-up
while a later member had waiting_input went to the wrong bot or started
a fresh run on the first member instead of continuing the ask.
Resolve the target bot like group sends: prefer waiting ask/takeover on
any member, then the newest steerable active run, then @mentions, then
the default first member. Extract followUpGroupThreadMessage and add
regression tests.
Co-authored-by: cynarAI <cynarAI@users.noreply.github.com>
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