fix(messages): validate send metadata object

[aiirvizionz]

Summary

• reject non-object send-message metadata before database work
• keep the existing default empty metadata object when metadata is omitted
• add focused regression coverage for invalid metadata payloads

Test

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

uGig billable bugfix candidate. Not counted as revenue until accepted, invoiced, and paid.