chore: add comments to make it clear how it works

Author: amanabiy

src/internal/utils/__tests__/flatten-children.test.tsx

@@ -26,8 +26,10 @@ describe('flattenChildren', () => {
     </Fragment>
   );
 
+  // Tests React 19+ behavior: uses Children.toArray() which does NOT flatten fragments
   describe('React 19+', () => {
     beforeEach(() => {
+      // Mock React.version to trigger Children.toArray() code path
       Object.defineProperty(React, 'version', {
         value: '19.0.0',
         writable: true,
@@ -45,8 +47,10 @@ describe('flattenChildren', () => {
     });
   });
 
+  // Tests React 16-18 behavior: uses react-keyed-flatten-children which DOES flatten fragments
   describe('React 16-18', () => {
     beforeEach(() => {
+      // Mock React.version to trigger react-keyed-flatten-children code path
       Object.defineProperty(React, 'version', {
         value: '18.2.0',
         writable: true,