Issue #3624795: Saving a product created inline (IEF) fails with duplicate...

Issue #3624795: Saving a product created inline (IEF) fails with duplicate variation UUID after re-opening its inline form: stale Product::$defaultVariation copy is re-saved through the computed default_variation field

Closes #3624795 Reopening an inline-created product can leave the computed default_variation field pointing to a stale variation copy, whose inherited pre-save handling attempts a duplicate insert. Make the computed field's pre-save a no-op so variation persistence remains owned by the product's variations field.