fix adding children (#51213)

Summary:
See #51212 - children aren't updated correctly in an old arch native view using the interop layer under Fabric.

This is caused by the mountChildComponentView method not updating the view, only adding the new view to a list that will be used on the next update to mount the child.

This commit fixes this by adding the same pattern as in unmountChildComponentView where children are inserted directly if the underlying paperview is available in the adapter - otherwise it uses the mounting list as before.

#Closes 51212

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## Changelog:

[IOS] [FIXED] - fixed adding child views to a native view using the interop layer

Pull Request resolved: #51213

Test Plan:
**Previous output**:

<image src="https://github.com/user-attachments/assets/472b95e7-0921-46c9-be6a-f31759c0cd26" width="200px" />

**After fix**:

<image src="https://github.com/user-attachments/assets/554387cd-c264-483e-9c52-d9cd40b42601" width="200px" />

Reviewed By: sammy-SC

Differential Revision: D74471278

Pulled By: cipolleschi

fbshipit-source-id: 798f9e7be389359bd6e3aa1b6a6e9fb799fcb369

Author: chrfalch

packages/react-native/React/Fabric/Mounting/ComponentViews/LegacyViewManagerInterop/RCTLegacyViewManagerInteropComponentView.mm

@@ -170,10 +170,24 @@ - (void)prepareForRecycle
 
 - (void)mountChildComponentView:(UIView<RCTComponentViewProtocol> *)childComponentView index:(NSInteger)index
 {
-  [_viewsToBeMounted addObject:@{
-    kRCTLegacyInteropChildIndexKey : [NSNumber numberWithInteger:index],
-    kRCTLegacyInteropChildComponentKey : childComponentView
-  }];
+  if (_adapter && index == _adapter.paperView.reactSubviews.count) {
+    // This is a new child view that is being added to the end of the children array.
+    // After the children is added, we need to call didUpdateReactSubviews to make sure that it is rendered.
+    // Without this change, the new child will not be rendered right away because the didUpdateReactSubviews is not
+    // called and the `finalizeUpdate` is not invoked.
+    if ([childComponentView isKindOfClass:[RCTLegacyViewManagerInteropComponentView class]]) {
+      UIView *target = ((RCTLegacyViewManagerInteropComponentView *)childComponentView).contentView;
+      [_adapter.paperView insertReactSubview:target atIndex:index];
+    } else {
+      [_adapter.paperView insertReactSubview:childComponentView atIndex:index];
+    }
+    [_adapter.paperView didUpdateReactSubviews];
+  } else {
+    [_viewsToBeMounted addObject:@{
+      kRCTLegacyInteropChildIndexKey : [NSNumber numberWithInteger:index],
+      kRCTLegacyInteropChildComponentKey : childComponentView
+    }];
+  }
 }
 
 - (void)unmountChildComponentView:(UIView<RCTComponentViewProtocol> *)childComponentView index:(NSInteger)index