Send a templated SMS to one or more recipients. Provide the message text once in message_template using [placeholder] tokens, then supply each recipient's values in template_variables.

• Set a default sender at the top level; override it per recipient if needed.
• Up to 250 recipients per request.
• Returns a batch task_id plus per-message identifiers; partial failures are reported inline.

Authentication: stream token in the Authorization header (see Authentication in the API Overview).