Standardize frontend queries to use itemTypes instead of itemType for configured items

Summary

Clean up all frontend code to consistently use itemTypes (plural) instead of itemType (singular) when querying configured items in the AI Catalog.

Problem

Currently, the frontend code uses both itemType and itemTypes inconsistently when querying configured items. This creates confusion and technical debt that should be addressed before GA.

Related Discussion

This proposal came from the discussion in gitlab-org/ai-powered/workflow-catalog/team-tasks#29 (comment 2853228282)