fix(license): duplicate items in software license dropdown

[eduardomozart]

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

When navigating to the Affected items tab of a License, the dropdown to add new items (such as Computers) was failing to exclude items that were already linked to the license (Closes #24950).

This happened because the dropdown initialization did not pass the used array parameter during the AJAX request, which is required for dropdownAllItems.php to filter out already-linked assets. As a result, items like computers incorrectly reappeared as available to be added.

This PR fixes the issue by querying both glpi_items_softwarelicenses and glpi_softwarelicenses_users to build the used array and correctly passing it along in the $p array to the AJAX dropdown handler.

Screenshots (if appropriate):

[cconard96]

I see no indication that it is intended for GLPI to support only assigning a single copy of a license to an asset.

[eduardomozart]

You're right. I find it a bit odd, but I can see scenarios where it applies, like software that is licensed per user seat, like Microsoft RDP User (licensed per user) and Device (licensed per device) CALs, but GLPI already allows linking licenses for Users and Computer assets if required, so I don't really see a scenario where allowing multiple users and computer license assignments would make sense. Maybe GLPI licensing could support more complex scenarios, like multiple conditions for linking (e.g. allow linking a license to a Computer + User) and not allow duplicates in those cases.
I'll let this PR open so the GLPI team can discuss it, and we can adopt or close this PR altogether; it's fine by me.