Fix stale message copy actions

[Chel-Tbl]

Closes #202.

Summary

• Remove the custom memo comparator from message actions so the copy handler always sees the latest message.parts.
• Add a message.content fallback for older/migrated messages that do not have text parts populated.
• Show a failure toast when clipboard writes fail instead of always reporting success.

Verification

• node .\node_modules\prettier\bin\prettier.cjs --check 'src/app/(general)/_components/chat/messages/message-actions.tsx'
• .\node_modules\.bin\tsc.cmd --noEmit --pretty false
• SKIP_ENV_VALIDATION=1 node .\node_modules\next\dist\bin\next build