fix: make orphaned value repair query cursor compatible across Nextcloud versions

[vitormattos]

Summary

• replace fetchFirstColumn() usage in field-value mapper with fetchAll(\PDO::FETCH_COLUMN) for broader DB cursor compatibility
• keep behavior identical by still returning ordered distinct user UIDs as strings
• add a dedicated unit test to lock compatibility behavior and prevent regressions

Problem

Running occ maintenance:repair could fail at the "Repair orphaned profile field values" step with:

Call to undefined method OC\\DB\\ResultAdapter::fetchFirstColumn()