The standalone chat-session delete path pruned channel_chat_session_binding but
not channel_outbound_card_message. Both are keyed by chat_session_id with no FK
(MUL-3515 §4) and no reaper, so deleting a chat session left the card rows as
permanent orphans — the same no-FK-orphan class as the #4810 installation fix,
which already covers the workspace-delete / runtime-teardown / reclaim paths.
Add DeleteChannelOutboundCardMessagesBySession and call it in the same tx as the
binding prune; extend the delete-chat-session test to assert both are swept.
Follow-up nit from the #5103 review (Elon).
MUL-3937
Co-authored-by: multica-agent <github@multica.ai>
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