fix(desktop): avoid idle chat history re-renders

[hdd69]

Summary

Desktop's parent controller can re-render while chat is idle, for example from background status polling. Before this change, those parent renders passed fresh callback props into ChatView, rebuilt the assistant runtime adapter, and caused the mounted assistant-ui thread/chat history to re-render even when $messages had not changed.

This stabilizes the idle chat path by:

• memoizing ChatView
• stabilizing DesktopController -> ChatView callback props
• memoizing the assistant runtime adapter
• only updating the incremental external-store runtime adapter when the adapter object changes
• adding a regression test that a parent idle tick does not re-render mounted chat history

This is intentionally scoped to idle render isolation. It does not change the scroll-anchor logic.

Test Plan

• npm run test:ui -- src/app/chat/index.test.tsx
• npm run test:ui -- src/components/assistant-ui/streaming.test.tsx
• npm run type-check

[alt-glitch]

Duplicate of #38470 (same author, same four apps/desktop files, near-identical diff differing only by a blank line). Consolidating on #38470 as the canonical PR.

[hdd69]

Duplicate of #38470 (same author, same four apps/desktop files, near-identical diff differing only by a blank line). Consolidating on #38470 as the canonical PR.

This is true indeed. It was done per mistake and I apologize for that. Closing this one.