fix(messages): validate encrypted content type

[aiirvizionz]

Summary

• validate message recipient encrypted_content before base64 / serialized Buffer processing
• return a controlled error for corrupt non-string encrypted content instead of throwing while handling the failure path
• add regression coverage for malformed encrypted content returned from the database

Test

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

uGig billing

• Expected invoice: 0.50 USD if accepted/merged
• Counted as revenue now: no, pending review/payment