feat(lpu): track optional ComputerSystem part_number

[iExalt]

Executive Summary

• ComputerSystem.part_number was commented out in ad1b4d4 because Lenovo systems reported null for this field.

Summary

Expose ComputerSystem.part_number as an optional field so callers can read part numbers while correctly handling Lenovo systems that return null.

Root cause

The property had been commented out because the original required String type could not deserialize Lenovo's null value.

Validation

• mise exec -- cargo test model::system
• mise exec -- cargo fmt --check
• git diff --check