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Changes proposed in this Pull Request:

Closes #30618.

This PR seeks to resolve the errant stock adjustment on restocked items in partially refunded orders by preserving the _reduced_stock and _restock_refunded_items order item metadata value so wc_maybe_adjust_line_item_product_stock() correctly identifies no $diff between the ordered quantity and the amount deducted and restocked.

I've added unit test and E2E test coverage, but I'm still unsure of all of the potential ramifications of keeping the _reduced_stock metadata around (perhaps extensions are looking for it?).

How to test the changes in this Pull Request:

1. Create an order for 2 products with set stock levels (products with manage stock enabled and an inventory quantity)
2. Refund one of the products on the order and select the "restock refunded items" option.
3. Notice the note is added confirming the stock has been restored.
4. Update the order (click "update").
5. Verify there is not a new note indicating the stock level has been reduced again.
6. Verify the product inventory levels have not changed (beyond the restocking) by checking the product edit screen.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

Fix - errant stock adjustment on restocked items when saving partially refunded orders.

FOR PR REVIEWER ONLY:

• I have reviewed that everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities. I made sure Linting is not ignored or disabled.