Creates a sender registration and sets status to PENDING_APPROVAL.

• If the account does not use parent/child accounts, the API ignores child_account_id and registers the sender for the authenticated account.
• If the account is a parent account and you omit child_account_id, the API registers the sender for the parent account.
• If you provide child_account_id, the API registers the sender for that child account.
• If a registration for the same sender was already created within the last 30 minutes (for the same account or child account), the API returns 429 Too Many Requests. This prevents abuse of the verification retry limit.
• Any existing PENDING_APPROVAL registration for the same sender (for the same account or child account) is automatically cancelled before the new one is created.