fix: Bug in fetchIdentities when using first()

[b1tc0re]

Description This PR fixes a critical bug when using withIdentities()->first() and improves test reliability.

Fixed identity loading bug:

• Added missing ID population for first() method in fetchIdentities
• Resolved "undefined array key id" error when calling withIdentities()->first()
• Now correctly handles identity relationships for single results

Checklist:

• [ ] Securely signed commits
• [X] Component(s) with PHPDoc blocks, only if necessary or adds value
• [X] Unit testing, with >80% coverage
• [ ] User guide updated
• [X] Conforms to style guide

[michalsn]

Thanks! However, there's no bug per se - methods like withIdentities(), withGroups(), and withPermissions() are intended to be used with the find* methods.

That said, we're always open to improvements. If you're interested, feel free to update all the relevant methods and documentation accordingly.