Demonstrates sending SMS to large recipient lists.

The sendBulk() method handles batching automatically:

• Splits into batches of 200 (API maximum per request)
• 0.5 second delay between batches (rate limiting)
• ERR013 (queue full) retried up to 3 times with exponential backoff (30s, 60s, 120s)

• OK: all batches succeeded
• PARTIAL: some batches succeeded, some failed
• ERROR: all batches failed

Always check getInvalid() for phone numbers that failed local validation.

• Estimate credit cost: recipients x pages per message
• Check cached balance: sms.getCachedBalance()
• Use validate() to pre-check numbers
• Call coverage() at startup and cache prefixes for local country validation