[Fix] Prevent stale nested field values when re-adding items in SimpleFormIterator

[WiXSL]

Problem

SimpleFormIterator reused values from a removed item when adding a new one back, for nested fields.

Solution

• Create proper default values for nested fields when adding a new item
• Read iterator row values from the current form state so removed items are not reused

How To Test

1- Open a form with an ArrayInput
2- Use nested sources
3- Remove an item
4- Add a new item
5- Verify the new row is empty and does not reuse the removed values

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The PR includes unit tests (if not possible, describe why)

[ThieryMichel]

Why do you need this ? It adds extra rerenders, and removing it do not break any tests, nor does it reintroduce the bug.
The fix in SimpleFormInterator seems sufficient.

[WiXSL]

You are right! Based on my earlier testing, I thought this change was needed. I re-ran the tests now and the fix still works without it, so we can avoid those extra re-renders.