Send a pre-approved WhatsApp template message to a contact. Use this endpoint to
initiate a conversation outside the 24-hour customer service window — for example,
notifications, marketing campaigns, or re-engagement messages. To reply within an
active 24-hour session, use sendSessionMessage instead.

Behavior:

• The template_name must reference a template that has been approved by WhatsApp.
  Requests referencing pending or rejected templates return 400.
• parameters are positional: the order in the array must match the placeholder
  order in the approved template body (e.g., {{1}}, {{2}}, …).
• broadcast_name is used for grouping and analytics in WATI dashboards; it does
  not affect message delivery.
• channel_number routes the message through a specific WhatsApp Business Account
  when the tenant has multiple WABAs connected.
• A 200 response means the message was accepted by WATI, not delivered yet.
• To detect delivery failures, listen to the Template Message Failed webhook.
• To detect successful delivery, listen to the Delivered, Read, or Reply
  webhooks.
• Marketing-category templates are subject to per-tenant daily limits; once the
  limit is reached, subsequent calls return 400 until the next window.
• The endpoint returns 400 when the tenant is in offline mode or when the
  trial-plan broadcast quota is exhausted.