Fix key Warning for Flattened Positional Children (#29662)

## Summary

#29088 introduced a regression
triggering this warning when rendering flattened positional children:

> Each child in a list should have a unique "key" prop.

The specific scenario that triggers this is when rendering multiple
positional children (which do not require unique `key` props) after
flattening them with one of the `React.Children` utilities (e.g.
`React.Children.toArray`).

The refactored logic in `React.Children` incorrectly drops the
`element._store.validated` property in `__DEV__`. This diff fixes the
bug and introduces a unit test to prevent future regressions.

## How did you test this change?

```
$ yarn test ReactChildren-test.js
```

DiffTrain build for [72644ef](72644ef)

compiled/facebook-www/REVISION

@@ -1 +1 @@
-38e3b23483bf7a612391cd617a8926aa1f3cf52e
+72644ef2f2ec7a274f79f6b32320d62757521329

compiled/facebook-www/React-dev.classic.js

@@ -22,7 +22,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-var ReactVersion = '19.0.0-www-classic-5a597884';
+var ReactVersion = '19.0.0-www-classic-f4a35977';
 
 // Re-export dynamic flags from the www version.
 var dynamicFeatureFlags = require('ReactFeatureFlags');
@@ -1799,9 +1799,16 @@ function createElement(type, config, children) {
   return ReactElement(type, key, ref, undefined, undefined, getOwner(), props);
 }
 function cloneAndReplaceKey(oldElement, newKey) {
-  return ReactElement(oldElement.type, newKey, // When enableRefAsProp is on, this argument is ignored. This check only
+  var clonedElement = ReactElement(oldElement.type, newKey, // When enableRefAsProp is on, this argument is ignored. This check only
   // exists to avoid the `ref` access warning.
   enableRefAsProp ? null : oldElement.ref, undefined, undefined, oldElement._owner, oldElement.props);
+
+  {
+    // The cloned element should inherit the original element's key validation.
+    clonedElement._store.validated = oldElement._store.validated;
+  }
+
+  return clonedElement;
 }
 /**
  * Clone and return a new ReactElement using element as the starting point.

compiled/facebook-www/React-dev.modern.js

@@ -22,7 +22,7 @@ if (
 ) {
   __REACT_DEVTOOLS_GLOBAL_HOOK__.registerInternalModuleStart(new Error());
 }
-var ReactVersion = '19.0.0-www-modern-c9630b74';
+var ReactVersion = '19.0.0-www-modern-85c20051';
 
 // Re-export dynamic flags from the www version.
 var dynamicFeatureFlags = require('ReactFeatureFlags');
@@ -1802,9 +1802,16 @@ function createElement(type, config, children) {
   return ReactElement(type, key, ref, undefined, undefined, getOwner(), props);
 }
 function cloneAndReplaceKey(oldElement, newKey) {
-  return ReactElement(oldElement.type, newKey, // When enableRefAsProp is on, this argument is ignored. This check only
+  var clonedElement = ReactElement(oldElement.type, newKey, // When enableRefAsProp is on, this argument is ignored. This check only
   // exists to avoid the `ref` access warning.
   enableRefAsProp ? null : oldElement.ref, undefined, undefined, oldElement._owner, oldElement.props);
+
+  {
+    // The cloned element should inherit the original element's key validation.
+    clonedElement._store.validated = oldElement._store.validated;
+  }
+
+  return clonedElement;
 }
 /**
  * Clone and return a new ReactElement using element as the starting point.