* Fix split-tests loading in React component: use `useSplitTestContext` instead of `getSplitTestVariant`
* Replace use of `isSplitTestEnabled` by `useSplitTestContext`
* Add SplitTestProvider to roots, and fix tests
* Create `useFeatureFlag` hook
* Use `useFeatureFlag` where applicable
GitOrigin-RevId: 9ff7bb3975d50bc4d07d74d93c482d56dc96f615