Use public unstable_TextAncestorContext API

React Native 0.81 exposed TextAncestorContext as a public API
(react/react-native#52368). Switch from the deep import path
'react-native/Libraries/Text/TextAncestor' to the public
'unstable_TextAncestorContext' export.

The deep-import target file in React Native exists solely as a
backwards-compatibility shim for react-strict-dom — see
https://github.com/facebook/react-native/blob/main/packages/react-native/Libraries/Text/TextAncestor.js
which carries a TODO from @huntie to delete it once this cross-repo
reference is fixed.

Same React Context object, no behavioral change. Test mock updated
to match the new import path; old deep-import mock deleted.

Author: Yamin Yassin

packages/react-strict-dom/src/native/react-native/TextAncestorContext.js

@@ -7,5 +7,5 @@
  * @flow strict-local
  */
 
-import TextAncestorContext from 'react-native/Libraries/Text/TextAncestor';
+import { unstable_TextAncestorContext as TextAncestorContext } from 'react-native';
 export { TextAncestorContext };

packages/react-strict-dom/tests/__mocks__/react-native/Libraries/Text/TextAncestor.js

@@ -120,6 +120,8 @@ export const View = 'View';
 
 export const experimental_LayoutConformance = 'LayoutConformance';
 
+export const unstable_TextAncestorContext = 'TextAncestorContext';
+
 export const useColorScheme = jest.fn().mockReturnValue('light');
 
 export const useWindowDimensions = jest