chore(model-selector): remove redundant z-index hacks after overlay unification

Now that base/ui primitives carry z-[1002] by default (#33185),
the per-call-site overrides (z-[1002] on ModelSelector, z-[1003]
on nested PopupItem dropdown) are no longer needed — DOM order
handles stacking for same-z-index portals.

web/app/components/header/account-setting/model-provider-page/model-selector/index.tsx

@@ -11,7 +11,6 @@ import {
   PopoverContent,
   PopoverTrigger,
 } from '@/app/components/base/ui/popover'
-import { cn } from '@/utils/classnames'
 import { useCurrentProviderAndModel } from '../hooks'
 import ModelSelectorTrigger from './model-selector-trigger'
 import Popup from './popup'
@@ -85,16 +84,10 @@ const ModelSelector: FC<ModelSelectorProps> = ({
           </button>
         )}
       />
-      {/*
-       * TODO(overlay-migration): temporary layering hack.
-       * Some callers still render ModelSelector inside legacy high-z modals
-       * (e.g. code/automatic generators at z-[1000]). Keep this selector above
-       * them until those call sites are fully migrated to unified base/ui overlays.
-       */}
       <PopoverContent
         placement="bottom-start"
         sideOffset={4}
-        className={cn('z-[1002]', popupClassName)}
+        className={popupClassName}
         popupClassName="overflow-hidden rounded-lg"
         popupProps={{ style: { minWidth: '320px', width: 'var(--anchor-width, auto)' } }}
       >

web/app/components/header/account-setting/model-provider-page/model-selector/popup-item.tsx

@@ -148,13 +148,7 @@ const PopupItem: FC<PopupItemProps> = ({
               </div>
             )}
           />
-          {/*
-           * TODO(overlay-migration): temporary layering hack.
-           * This nested provider-settings dropdown opens from inside ModelSelector
-           * (z-[1002]), so it must stay one level higher until all related modals
-           * are unified on the new overlay stack.
-           */}
-          <PopoverContent placement="bottom-end" className="z-[1003]">
+          <PopoverContent placement="bottom-end">
             <DropdownContent
               provider={currentProvider}
               state={state}