Send a batch of emails

Sends 1 to 100 separate emails in one call, like Resend's batch: each email has its own to (and its own text), so everyone sees only their own email, for example a personal invitation per person. Each email follows the rules of POST /emails, except that a batch has no attachments (send those one by one). All or nothing: every email is checked first, and if one is refused, none is sent and the error says which (index). Every email and every recipient counts toward the limits and the bundle; the whole batch is one call for the per-minute limit. An Idempotency-Key (header or idempotency_key) returns the same emails instead of sending them again. With a test key nothing is sent (status simulated).