fix(sms): handle missing Twilio credentials

[rissrice2105-agent]

Summary

• makes validateTwilioCredentials() return a structured validation error for missing/non-object input instead of throwing
• adds a regression test for null credential config
• preserves existing validation behavior for valid Twilio credentials, invalid SID/token, phone format, and messaging service SID

Fixes #107.

Validation

• node --test tests/twilio-validator.test.js - 16 passing
• direct Node check for validateTwilioCredentials(null) returning { valid: false, errors: [...] }