Work items list [FE] - Add import CSV

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What does this MR do and why?

Add the ability to import work items from CSV files with the following changes:

• Create WorkItemsCsvImportModal component with file upload functionality
• Add CSV import button to WorkItemListActions dropdown
• Support feature flag toggling between work items and issues terminology
• Include proper GraphQL mutation handling with file upload context
• Add comprehensive test coverage for import modal and list actions
• Implement proper error handling and success notifications
• Add file size validation and user guidance text

The import modal validates CSV format requirements and provides background processing with email confirmation upon completion.

References

Relates to #536879 (closed)

Screenshots or screen recordings

Screen_Recording_2025-08-26_at_3.42.31_PM

Steps to verify

• Make sure to enable work_items_project_issues_list
• Go to project issues
• Click on actions > Import CSV
• On Modal use Example_CSV.csv
• Check mail in http://127.0.0.1:3000/rails/letter_opener/
• Once received, reload and verify

Related to #536879 (closed)