fix(conversations): validate chat participant ids

[aiirvizionz]

Summary

• trim participant IDs before duplicate checks and inserts
• reject null, non-string, and blank participant IDs with a 400 response
• add focused regression coverage for malformed participant arrays

Verification

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

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