Resolve "Work items list [FE] - Add export CSV"

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What does this MR do and why?

A split from !199945 (merged) for export CSV only

References

Screenshots or screen recordings

Before	After
	Screencast from 11-08-2025 01:26:12.webm

How to set up and validate locally

1. In rails console enable the feature flag Feature.enable(:work_item_planning_view) and Feature.enable(:work_items_project_issues_list)
2. Visit any project pages such as http://gdk.test:3000/toolbox/gitlab-smoke-tests/-/work_items

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #536879 (closed)