MAINT Extract message view model from MessageList

[Isaac Emmanuel (1cbyc)]

Description

Refs #2654.

MessageList currently owns pure message-shaping rules alongside its React rendering. This extracts the first requested boundary into a typed messageViewModel module:

• preserve explicit display-piece ordering, identities, score ownership, and test IDs
• map legacy content and attachments without coupling those rules to the component
• isolate structured JSON formatting from presentation

MessageList remains responsible for list rendering, loading/empty states, and scrolling. This is intentionally a focused first slice of the issue; it does not change user-visible behavior or attempt the separate media, score, and action component boundaries.

Tests and Documentation

• npm test -- --runInBand src/components/Chat/messageViewModel.test.ts src/components/Chat/MessageList.test.tsx (67 passed)
• npm run type-check
• npx eslint src/components/Chat/MessageList.tsx src/components/Chat/messageViewModel.ts src/components/Chat/messageViewModel.test.ts --max-warnings 0

Added focused tests for structured-content formatting, explicit display-piece identity/order, and legacy content/attachment score separation. No documentation changes are needed because this is an internal refactor with unchanged behavior.

[Roman Lutz (romanlutz)]

Thanks for the contribution Isaac Emmanuel (@1cbyc) ! We've had a lot of cases recently where the CLA wasn't accepted and authors never responded after the initial PR push. If I don't hear back by 10/17 I'll close this PR because we need the CLA. See posts by @microsoft-github-policy-service above.

[Isaac Emmanuel (1cbyc)]

@microsoft-github-policy-service agree

[Isaac Emmanuel (1cbyc)]

Roman Lutz (@romanlutz) kindly check again.