Apply the connect-invite at-most-once pattern to mirrored DMs, group
posts, channel invites, and intros. Provider send errors now keep the
outbox row claimed as sent without a handle instead of reverting to
pending and risking duplicate delivery.
Pre-send setup failures (thread resolution) still retry with backoff.
Pass outbox idempotency keys through to sendToThread for future provider
dedupe.
Co-authored-by: cynarAI <cynarAI@users.noreply.github.com>
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