fix(messages): handle malformed chat message JSON

[aiirvizionz]

Summary

• return a controlled 400 response for malformed legacy chat message JSON bodies
• prevent corrupt request bodies from falling through to the generic 500 handler
• add regression coverage that stops 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