SearchKit - Fix multi-valued afform filters

[colemanw]

Overview

Fixes a SearchKit regression causing multi-select afform filters to stop working.

Before

• Create a search for e.g. Activities
• Add it to a Form
• Add a multi-select filter e.g. Activity Type
• Visit the form and try using the filter. It will return no results.

After

Fixed, test coverage added.

Technical Details

Multi-select search filters stopped working as of 007167d due to the label lookup assuming single-valued input. This fix adds a test for getting labels with multi-valued filter input.

[civibot]

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[seamuslee001]

This seems fine to me merging