fix(appconfig-userpreference): Fix doc types for returned arrays

[nickvergessen]

Assisted-by: ClaudeCode:claude-opus-5

• Drive by while trying to debug Notification emails are still sent to disabled users since Nextcloud 34 (false === 'false' in MailNotifications) notifications#3338 for fix(mail): Correctly skip disabled user email notifications again notifications#3339
• But Psalm still does not turn red until <DocblockTypeContradiction errorLevel="error"/> is added as an IssueHandler sadly. I think it's still worth adding.
• Maybe it would be even better to have *String, *Bool, … methods for all of them, so default psalm notices the mistakes?

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI