fix(messages): reject invalid encrypted content values

[aiirvizionz]

Summary

• reject non-string encrypted_contents values before participant/database work
• reject blank encrypted payload strings before Buffer.from conversion
• add focused regression coverage for both malformed cases

Verification

• node node_modules\vitest\vitest.mjs run --config %TEMP%\qryptchat-vitest-minimal.config.mjs src/app/api/chat/messages/route.test.js
• git diff --check

This is a focused hardening fix for the accepted uGig QryptChat testing gig.