Fix positions not reset between rack items when checking model update

[Herafia]

Checklist before requesting a review

Please delete options that are not relevant.

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
• This change requires a documentation update.

Description

• It fixes !44135
• When updating a DC model's required_units, $positionsToCheck was initialized outside the foreach loop, causing positions from previous rack items to accumulate and trigger false-positive conflicts on unrelated racks. Moving the initialization inside the loop ensures each rack item is checked independently.

Screenshots (if appropriate):

[Rom1-B]

Can you add test?