Send up to 10,000 messages in a single API call. Each message in the batch
is validated independently — some may be accepted while others are rejected.

The total cost is reserved from your balance upfront. If the batch would
exceed your balance, the entire batch is rejected.

Each accepted message gets its own message_id for individual tracking.
Messages inherit default values for any omitted optional fields.

Idempotency: Use the X-Idempotency-Key header to make the entire
batch idempotent.