Stabilize composer model picker rerenders

[cursor]

What Changed

• Stabilized the ChatComposer -> ProviderModelPicker onOpenChange callback with useCallback.
• Keeps the memoized provider/model picker from receiving a fresh inline function on every composer prompt render.

Why

React Doctor found broad React Compiler/performance backlog in apps/web, and a manual hot-path pass found the composer footer model picker rerendering during prompt-only typing. React Scan baseline recording showed ProviderModelPicker increasing with every composer keystroke (20 -> 31). After this change, the same recording kept ProviderModelPicker flat at 5 while the actual editor components continued to update.

React Scan recordings included:

• Before: react_scan_provider_picker_before.mp4
• After: react_scan_provider_picker_after.mp4

UI Changes

No visual UI changes. This is a render-performance fix demonstrated with React Scan before/after recordings.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Validation:

• npx react-doctor@latest apps/web --verbose --no-telemetry (scan completed; existing backlog reported)
• npx react-scan@latest --help (CLI checked; this environment exposes init, so recordings used React Scan browser instrumentation)
• vp check
• vp run typecheck

  

Note

Stabilize model picker onOpenChange callback in ChatComposer

Wraps the inline onOpenChange function passed to the model picker in useCallback so it has a stable reference across renders. State update behavior is unchanged.

^{Macroscope summarized 2bda93b.}