Validates the durable SMS render contract.
Only the message body is a render concern. Sender ID, Messaging Service SID, and recipient phone number are adapter-config territory — never in render_data.
GSM-7 encoding supports up to 160 characters per segment; UCS-2 (unicode) supports up to 70 characters per segment. Multi-segment messages are billed per segment. Segmentation and encoding are vendor concerns handled in the adapter, not validated here.