fix(playground): prevent model group option stretching (#6120)

* fix(playground): prevent model group option stretching

- keep group options at a fixed 2rem height and align them to the top.
- organize layout code in a dedicated module and cover layout and scrolling behavior.

* docs(web): strengthen frontend testing requirements

- require regression coverage for behavior changes, bug fixes, and UI states.
- define module-scoped test organization, stable assertions, mocks, and verification rules.

web/src/components/model-group-selector.tsx

@@ -65,7 +65,7 @@ import { cn } from '@/lib/utils'
 import {
   modelGroupSelectorLayoutClasses,
   scrollSelectedOptionIntoView,
-} from './model-group-selector-layout'
+} from './model-group-selector/layout'
 
 interface ModelOption {
   label: string