Change a property’s fallback_value.

Makes this call safe to retry. Send the same key with the same body and the original response is replayed instead of the work happening twice. Keys are 1-256 characters and are remembered for 7 days. While the first attempt is still running, a second call with that key returns 409 idempotency_in_flight (Resend calls this concurrent_idempotent_requests); the same key with a different body returns 409 idempotency_payload_mismatch (Resend: invalid_idempotent_request). Resend-Idempotency-Key is accepted as the same header.

Alias of Idempotency-Key. Idempotency-Key wins if both are sent.