fix(messages): reject encrypted contents arrays

[aiirvizionz]

Summary

• reject encrypted_contents arrays in the legacy chat message send endpoint
• keep the fan-out input constrained to a user-id to encrypted-content object map
• add regression coverage that blocks array payloads before participant or message creation work

Test

• 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

uGig billing

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