[FIX] mrp: fix MO onchange issue with move_finished_ids

Previous fix commit 5e34a02c was too aggressive in when it would delete
the move_finished_ids. In certain use cases it would result in no
move_finished_ids and a corrupted MO:

Steps to reproduce:
1. Create a new MO
2. Save the MO (do NOT confirm)
3. Update the qty to product_qty (qty to produce)
4. Confirm + Mark As Done

End result: "qty to produce must be positive" error whenever MO was
attempted to be completed and MO can never be completed.

To fix this, we split out when the move_finish_ids. They should all
be deleted ONLY when the product to produce is changed. Unfortunately to
cover all cases, we must always wipe the moves whenever the product is
changed (e.g. when changing the product twice with the original product
being the final saved value, we have no way of knowing to keep the
original move_finished_ids due to onchange only being able to check
against the last saved value, not last selected value).

Additional test + test update done to support preventing this
catastrophe in the future.

Part of Task: 2618962

closes odoo/odoo#74831

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>