Send one catalogued Slack message as Patricia

Posts one catalogued message, built from dummy data, into a public channel of the ONE allowlisted workspace. Nothing in it is clickable and every link in its text is defused. The request cannot name a workspace. Inspect errors[0].type on a 409 for the specific refusal (wrong_workspace, not_public, shared, archived, is_dm, mismatched_record, not_in_channel, partial_send, delivery_unknown, idempotency_conflict, send_in_progress). The required Idempotency-Key belongs to one principal and one message/channel pair. Repeat it to replay the recorded receipt or failure without posting another copy. An unsettled claim never re-sends. Only a proven-not-posted refusal releases the key, an unreachable Slack among them; a new key permits a deliberate second post.