Allow image replies in existing group temporary conversations while preserving passive-only restrictions. Missing, expired, or mismatched sessions remain rejected before media processing, and sending never creates or refreshes a session.
Fixes#471
Replace the open message bag with a validated discriminated union and preflight complete messages before uploads or sends. Return BAD_REQUEST for invalid or unsendable segments instead of dropping fields, sending partial content, or silently changing private messages.
send_private_msg / send_msg can now reply to a group temp-session message
by passing the source group_id. Inbound temp messages surface their
source group on sender.group_id; a reply is only made into a temp session
the bot has an inbound record of. Group temp replies route through
RoutingHead.grpTmp (field 3).
DrSmoothl