Post-rebase composition over #99431/#99429/#99427: file-attachment sends
route through _run_message_send so the mention-recovery ladder covers
media captions; _send_file_attachment/_send_local_file honor the
resolved thread-root anchor and reply_to_mode opt-out; send() records
event_meta on the verified receipt id (#75826); test fakes gain the
auth_tag kwarg and accepted-receipt shape.