Reduce Twilio SMS costs with GSM/UCS character validation

Customer SMS sent through the Admin Toolkit had no enforced character limit. Messages crossing GSM/UCS‑2 segment boundaries silently split into multiple billed segments, driving up the Twilio bill.

Enforce correct SMS character limits on the backend for both GSM and UCS‑2 (Unicode) encodings, so messages stay within intended segment counts and costs drop.

Added backend validation that accounts for the encoding (GSM vs UCS‑2) and enforces the appropriate per‑segment character limit before an SMS is sent, blocking messages that would otherwise fragment into extra billed segments.

Oversized messages are caught before sending, keeping SMS within intended segment counts and reducing Twilio costs - a direct, recurring saving on every customer message.