Send a test message through the real receiving path

Requires a full key or a dashboard session with write access. Sending, read_only and inbox keys are forbidden. A foreign, deleted or other-mode inbox is not found. The domain must have verified receiving (422 domain_receiving_disabled otherwise). Missing platform mailer settings return 503 unavailable. The platform tenant sends live, including to test inboxes. The fixed body contains code 123456 and one link. The subject is "Sendtier test message <8-char test id>". Acceptance does not activate Inbox. Limits in any rolling 60 minutes are 5 per recipient address, 20 per tenant and 300 platform-wide; 429 rate_limited includes Retry-After. Idempotent replays return the stored result without another send or debit. Pass wait_after as after to messages/wait, with subject_contains set to subject. This opaque (received_at, id) boundary is captured before submission, works on an empty inbox and can be reused after 204 responses.